Note: Descriptions are shown in the official language in which they were submitted.
113~57S
The present invention relates to an improvement
of the apparatus for the desalination of both sea water and
brackish water as disclosed in the U.S. patent N 3,961,658
issued June 8, 1976 to Pagani.
The apparatus according to the above cited patent
operates accordlng to a multiple stage method, for the
detailed description of which reference is invited to the
specification of the parent patent, said method comprising
the use of one or more vertical cylindri~al columns which are
partitioned into a number of stages, each stage consisting of
two liquid-film vertical evaporators having a cross-sectional
outline in the form of a cricular segment and having, between
the two bodies a preheater for the salt water which is ar-
ranged horizontally.
A stage formed in the manner recalled above is suit-
able for installations which have a from average to high
output, whereas for installations of small output, an apparatus
such as the one disclosed in U.S. patent N 4,062,734 issued
on December 13, 1977 to Pagani, may be employed.
The latter Patent provides an embodiment of the tube
bundle of each stage as an entity having the tube plate with
a circular cross-section and the preheater, which is horizontal,
inserted into a through-tube which diametrically passes through
the brackish-water tub.
In installations producing high output, it has now
been found that the vertical arrangement for the preheaters
as well as the evaporators involves surprising advantages over
those provided by U.S. patent N 3,961.658 issued June 8, 1976,
inventor Giorgio Pagani~both from the point of view of the
invested capital and the overall electricity usage.
According to the present invention there is provided
an improved multiple effect distillation apparatus for the
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desalination of sea water wherein a column of superposed
cylindrical sections are provided, each section having the
following elements: at least one falling film evaporator
disposed in each said section other than the lowermost section,
each said evaporator having a bundle of vertical tubes; a
collection tub within each said section, with the tub in the
uppermost section being connected at its upper end to a
brackish water feed tube with the remaining tubs in the
succeeding lower sections being connected at their upper
ends to said vertical tubes of the film evaporator, disposed
in the section which is immediately superjacent, and wherein
the bottom of the tub in the lowermost section is connected
to a brackish water discharge tube, and with the bottoms of
the tubs of each remaining section being connected to the
upper end of the vertical tubes of the associated film
evaporator of that section; a restriction means placed on
the bottom of each tub, said restriction means for permitting
the flow of brackish water towards the underlying evaporators
and dissapating the positive pressure difference existing
between the tub and the evaporator; openings formed in the
upper portion of each tub in each section other than the
uppermost section, said openings to allow vapor to flow from
each said tub about the associated evaporator of that section;
at leastone siphon tube for withdrawing the condensate collected
at the bottom of each section and for reintroducing said
condensate at an intermediate point of the succeeding subjacent
section; means for transferring any inext gases from each
section to a vacuum system for collecting said inert gases;
means for introducing sea water to the tub of the uppermost
section and means for introduction of steam to the exterior
surface of said film evaporator of said uppermost section;
means for removing condensate from the lowermost section
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wherein said improvement comprises; preheating means, said
preheating means including a plurality of longitudinally -
extending, cylindrical ducts disposed in a substantially
vertical orientation, with one of said ducts being located
within each section of said apparatus and oriented in end to
end, colinear relationship, all of said ducts being in com-
munication to define a flow path, such that brackish water
may be passed through said ducts prior to its introduction
to the uppermost section of said apparatus whereby the vapors
in each section functions to elevate the temperature of the
brackish water within the ducts of the preheating means to
facilitate its vaporization within said apparatus.
Embodiments of the present invention will now be
briefly described, this disclosure being intended as an
explanation rather than a limitation of the invention.
Objects and advantages of the subject invention
will become apparent in light of the specification taken in
conjunction with the accompanying drawings in which:
Figure 1 is a cross-sectional view of one chamber
of a desalination apparatus taken along the line B-B in
Figure 2 and illustrates the preheating means of the subject
invention.
Figure 2 is a cross-sectional view of a desalination
apparatus taken along the line A-A in Figure 1 and illustrating
the location of the preheating means of the subject invention.
With reference to FIGURE 1, which shows a longitu-
dinal cross-sectional view of a column trunk a single desalina-
tion stage is described.
The single chamber illustrated in Figures 1 and 2
is part of a desalination apparatus which is similar to the
desalination apparatus described in U.S. Patent No. 3,961,658,
with the differences therebetween being discussed in detaii
1~3657S
hereinafter. A single chamber of the multi-chambered
desalination apparatus comp~ises the following parts:
at least one vextical film evaporator 1, having a
plurality of tubes lA (only one tube is illustrated).
At least one brackish water collecting tub 2 is
provided in each section having its bottom end connected
to the top of the evaporators 1. The tubs 2 in the lowermost
cylindrical chamber of the apparatus (not shown) are connected
to a brackish water removal means such as a discharge tube.
The tubs 2 in the uppermost section of the apparatus (not shown)
are connected to a brackish water feeding tube.
A lamination system is provided in each tub 2 which
essentially divides the tub into an upper section and a lower
tub section 4. The lower second tub section 4 is connected
to the top tube plate 8 of the underlying film evaporator 1.
The lamination system may consist of a plate with a narrowed
orifice as shown in U.S. Patent No. 3,961,658. The lamination
system 7 is intended to dissipate the pressure difference
existing between the tub 2 and the associated evaporator.
Openings 3 formed through the top portion of the
sidewalls of each tub 2 with the exception of that of the
uppermost section or chamber (not shown).
One or more siphon tubes 9 for drawing the condensates
collected at the bottom of each section to introduce the
condensates into intermediate points of the subsequent section
again.
Connection pipes 12 between a stage and the next one
to transfer the inert gases which are possibly present to the
vacuum pump.
The final condenser (not shown) placed beneath the
last stage, is of the horizontal tube bundle type with fixed
tube plates placed on diametrically opposite portions of the
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column, and condenses all the steam which is produced in the
last stage.
In accordance with the subject invention, a new
and improved preheating means is provided for preheating
brackish water prior to its introduction to the tubs 2 in
the uppermost section of the apparatus. By preheating the
brackish water, less energy is needed to vaporize the water
in the uppermost section. The new and improved preheating
means consists of a plurality of cylindrical ducts 5 disposed
in a substantially vertical orientation, with one duct being
located in each section of the apparatus. The ducts 5 are
in communication with each other and define a flow path for
the brackish water. The bottom of each duct 5 rests against
the plate 10 separating the sections of the apparatus. The
top of each duct 5 is connected to a thermal expansion joint 6
which, in turn, is connected to the upper plate 10 of each
respective chamber. The thermal expansion joint 6 is provided
to dampen the effects of varying temperatures in each section
of the apparatus, which cause the ducts 5 to expand at dif-
ferent rates.
As illustrated in Figure 2, each of the tubs 2 hasa configuration approximating a section of a circle. The
tubs 2 are oriented such that they occupy approximately 3/4ths
of the interior of the chamber. Each duct 5 is cylindrical
in configuration and occupies the remaining sector of the
chamber. By this arrangement, efficient preheating of the
brackish water is facilitated.
In use, sea or brackish ~ater is pumped through the
new and improved preheating means, such that it is preheated
by the vapors in the chambers prior to its introduction into
the tubs 2 of the uppermost section of the apparatus.
Conveniently, as heat is transferred from the steam to the
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brackish water in the ducts, the temperature of the water
vapor is reduced below the boiling point such that condensed
fresh water is produced. Fresh steam, which is also introduced
into the uppermost section of the apparatus, futhers heats the
brackish water as it passes from the uppermost tub through the
associated evaporators 1 and into the tubs 2 of the section
immediately subjacent. The heating of the brackish water causes
its partial evaporation, with the vapor passing into the
interior of the cylindrical chamber through the openings 3
provided in the tubs 2. The openings 3 may be provided with
demisters to prevent the espace of non-vaporized brackish water.
The non-vaporized brackish water passes through the lamination
7 into the lower tub 4 and thereafter into the subjacent set
of evaporators 1. The brackish water in the evaporators 1
is heated by the steam within the chamber. As the steam heats
the brackish water to its boiling point, the steam itself is
condensed into pure water.
The operation within each chamber is identical,
with the interior pressures decreasing from the uppermost
chamber to the lowest. The pure water condensate formed in
the bottom of each section is siphoned to the sectlon imme-
diately below through siphon tubes 9. In the lowermost
section, the remaining brackish water in the tubs 2 is removed
via a discharge tube. As noted above, any inert gases which
were produced are removed by a vacuum pump. Any steam
remalning in the lowermost chamber is condensed by a horizontal
condenser (not shown3 such that all the pure water may be
removed from the apparatus.
The vertical preheaters of the subject invention
have many advantages over the horizontal preheaters used
heretofor in the prior art. These advantages which are more
fully described hereinafter, include the reduction of the
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total weight of the apparatus, elimination of individual
manifolds for each preheater, and the reduction of the pump
size used to feed the brackish water to the apparatus. Further,
the above described vertical arrangement facilitates the
maintenance and replacement of the ducts.
Two examples will now be reported which have the
purpose of better illustrating the invention without limiting
its scope in any manner.
EXAMPLE 1
The starting data are:
Salinity of the sea water 35,000 ppm (Parts per million)
pH of sea water at 20C 8.1
Temperature of sea water 26C
Primary steam 3 atm and 135C
Production of fresh water 1,500 c.metres an hour
Temperature of the fresh
water 46C
Salinity of fresh water 15 ppm
The embodiment of the desalination unit with vertical
preheaters is especially interesting for such a high output
as can be seen in the following table which reports a comparison
between the main features of the desalination apparatus for
the two alternatives, i.e. with horizontal and with vertical
preheaters, the heating efficiency being the same for both.
Horizontal Vertical
preheaters preheaters
N. of stages 15 15
N of columns 2 2
Dia.lst column8,000 milli- 6.500 milli-
metres metres
Dia.2nd column 8,000 " 6,500 "
Height 1st column37,200 " 41,400 "
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Horizontal Vertical
preheaters preheaters
Height 2nd column 40,800 milli- 45,000 milli-
metres metres
Specific electricity useup 2.34 kWh/cu. 1,86 kWh/cu.
metre metre
In addition to the comparison made hereabove it
must be added that the horizontal preheaters would require tube
plates having a diameter of 2,450 mm each, with serious
attendant problems as regards their construction and the sup-
porting of the bundles in the interior of the stages, due to
their weight proper, for the tight seal problems for the flanges
-~ with regard toair seeping from the outside and, lastly, that
which is the most important aspect, the virtual impossibility
of manipulating bundles of such a size when slipping them into
and out of the columns for upkeep and repair operations.
The fact that with this invention the overall
length of the two columns is by 8,400 mm longer than in
the case of the horizontal preheaters is not such as to
give a deceptive impression inasmuch as the considerable
reduction of the diameter, along with dispensing with the
external manifolds for the preheaters, said manifolds being
compulsorily made of a previous material (Cu/ni 90/10) and
the connecting pipes also of a costly material has the result
of a considerable and actual savings in the initial cost, a
savings which is best seen in the subsequent example hereof.
EXAMPLE 2
The starting data are:
Salinity of the sea water35,000 ppm
pH of sea water at 20C 8.1
Temperature of sea water 26C
Primary steam 3 atm and 135C
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Fresh water production 535 cu.metres an hour
Fresh water temperature 46C
Salinity of fresh water 15 ppm
The following table clearly shows that the lesser
output of 535 cu.metres an hour, which can be regarded as a
medium-to-high magnitude, makes the advantages of the invention
less conspicuous than in the previous example but, this
notwithstanding, the savings of materials is still significant
as will be seen hereafter.
Horizontal Vertical
preheaters preheaters
N of stages 15 15
N! of columns 2 2
Dia. 1st column 4,700 mm 4,350 mm
Dia. 2nd column 4,700 mm 4,350 mm
Height 1st column 37,200 mm 39,800 mm
Height 2nd column 40,800 mm 44,00 mm
Specific electricity useup2.35 kWh/cu.1,86 kWh/cu.
metre metre
In this case the comparison between the two approaches
has been made closer and has permitted that the actual eco-
nomical advantage so achieved might be more accurately
evaluated.
The overall weight of the two evaporating columns
is reduced by about 33 metric tons and, of these, 5.5 tons
are of a costly material ~Cu/ni 90/10) and the remainder
is carbon steel coated by a costly material.
It should be added that in the case of a unit with
laterally withdrawable horizontal preheaters, an external
scaffolding must be provided outside the columns, with work
flats at every stage and hoists for removing and installing
the preheaters.
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By virtue of the approach afforded by this invention,
both the evaporating bundles and the preheaters can be with-
drawn vertically from the column heads so that it is sufficient
to cater for a hoist of proper force above the column tops
and this hoist must be capable of servicing both columns and
of resting on a platform fastened to the column heads.
By so doing, the savings in the scaffoldings relative
to the servicing outside the columns can be estimated to about
140 metric tons, and the savings relative to the erection
can be added thereto.
The powerful pump to be used for charging the evapo-
rators become cheaper since, if the rate of delivery required
is the same, the hydraulic head to be overcome is reduced
from 180 metres to 120 metres.
The foregoing has also an influence on the reduction
of the erection time, a factor which is not to be overlooked
on account of the present situation in the market.
In addition to the reduction of the weight of the
materials, which means a resultant considerable reduction
inthe initial costs, a savings is also achieved in the running
costs of the installation on account of the reduced usage
of electric power.
For a yearly output distributed over 330 working
days and for a cost of electricity of 30 It.lire per kWh
such a savings will be: (2.34 - 1.86) x 1,500 x 24 x 330 x 30 =
174,636,000 It. Lire yearly, for Example 1 and, (2.35 - 1,86) x
535 x 24 x 330 x 30 = 62,286,840 It. LIRE yearly for Example
2.
It is apparent from Example 2 as reported above that
this invention can be used with a vantage, as outlined above,
also to installations having a potential medium-high outputs
and, very presumably, already from outputs of from 350 to 400
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cu.metres an hour whereas it is virtually unreplaceable for
high outputs such as from 800 to 1,000 cu.metres an hoùr and
over: installations with so high potential outputs are
becoming ever and ever more widespread.
