System for diaphragm distillation

A system for diaphragm distillation of a liquid, for example for desalination of salt water, comprising a diaphragm, through which vapor, but not liquid can pass, and a condensation surface for condensing vapor having passed through the diaphragm, and necessary passageways. According to the invention, the system comprises one or several units, each of which comprises a plurality of pipes located one within the other, viz. a first (1) inner pipe, a second (2) pipe located outside thereof and consisting of a diaphragm of the aforesaid kind, and a third (3) pipe located outside the second (2) pipe, which first (1) and third (3) pipes are of a gas-tight material, that said unit is surrounded by a fourth (4) gas-tight pipe, where a first (5) passageway is formed by the first (1) pipe and a second (6), a third (7) and a fourth (8) passageway are formed between said pipes (1,2;2,3;3,4) in successive order in the direction from the inside outward. The said passageways (5,6,7,8) in radial order are intended to transport a warm medium (WW) for the supply of heat, to transport the liquid (SW) to be distilled, to condense gas having passed through the diaphragm (2) and to transport distillate (PW) which has been condensed, where the condensation of the gas takes place against a condensation surface, which consists of the passageway surface opposite to the diaphragm (2), and, respectively, to transport a cold medium (CW) for heat removal.

This invention relates to a system for diaphragm distillation. 
Systems for diaphragm distillation are previously known, for example for 
sea water desalination. One such system is known from SE-PS No. 8002233-8. 
In this patent a system is described to comprise a diaphragm, the 
properties of which are such that vapor is permitted to pass through the 
diaphragm while liquid cannot pass therethrough. In parallel with the 
diaphragm a condensation surface is located and, furthermore, heat 
exchangers are provided, the object of which is to maintain the liquid to 
be distilled warm and to maintain the condensation surface cooled by a 
colder liquid flowing past. 
The present invention has the object to combine in one and the same system 
diaphragm distillation and the heat exchange required for the distillation 
process. 
The invention implies that a simple and compact unit is obtained, which can 
be coupled together with additional units, each of which comprises 
diaphragm, condensation surface and heat exchanging surfaces. 
The present invention, thus, relates to a system for diaphragm distillation 
of a liquid, such as for desalinating sea water, comprising a diaphragm, 
through which vapor but not liquid is permitted to pass, and a 
condensation surface for condensing vapour having passed through the 
diaphragm, and necessary passageways. The invention is characterized in 
that the system comprises one or more units, each of which comprises a 
plurality of pipes located one within the other, viz. a first inner pipe, 
a second pipe located outside thereof and consisting of a diaphragm of the 
afore-mentioned kind, and a third pipe located outside the second pipe, 
the said first and the said third pipe being of a gas-tight material, that 
the said unit is surrounded by a fourth gas-tight pipe, and a first 
passageway is formed by said first pipe, and a second, a third and a 
fourth passageway are formed between said pipes in successive order in the 
outward from inside direction, and that said passageways in radial order 
are intended to transport a hot medium for heat supply, to transport the 
liquid to be distilled to condensate gas having passed through the 
diaphragm and to transport distillate having been condensed, which 
condensation of the gas takes place against a condensation surface, which 
consists of the passagway surface opposite to the diaphragm, and, 
respectively, to transport a cold medium for conducting away heat.

Diaphragm distillation is a method of effecting distillation at a 
relatively low temperature without overpressure or vacuum. In diaphragm 
distillation a difference in temperature is maintained between the liquid 
to be distilled and the surface, against which condensation of gas having 
passed through the diaphragm takes place. Heat, therefore, is supplied to 
the liquid to be distilled, and heat is removed from the condensation 
surface. 
FIG. 1 is a schematic view of a section through a unit according to the 
invention assembled of several pipes. In FIG. 1 only a part of the unit 
length is shown. One or several such units can be utilized together. 
Each unit comprises a plurality of pipes located one inside the other, viz. 
a first inner pipe 1, a second pipe 2 located outside the same, and a 
third pipe 3 located outside the second pipe 2. The unit is surrounded by 
a fourth pipe 4. The first pipe 1, the third pipe 3 and the fourth pipe 4 
are made of a liquid and gas-tight or impermeable material, for example 
plastic or metal. The pipes 1,3,4 should be made of a material which has 
good thermal conduction properties and is corrosion resistant. The second 
pipe 2 consists of a diaphragm, which in known manner is designed so as to 
permit vapor, but not liquid to pass therethrough. 
By the arrangement shown in FIG. 1, a first passageway 5 is formed by the 
first, inner, pipe 1. A second passageway 6,and a third and fourth 
passageway 7 and 8 are formed between said pipes 1 and 2, 2 and 3, and 3 
and 4, respectively i.e., in successive order from inside to outside. 
The said passageay 5, 6, 7 and 8 in radial order are intended to transport 
a hot medium for the supply of heat, to transport the liquid to be 
distilled, to condense gas having passed through the diaphragm 2, and to 
transport distillate having been condensed and, respectively, to transport 
a cold medium for the removal of heat. The said radial order can be from 
inside outward or from outside inward, because two passageways 5,6 are 
located inside of the diaphragm 2 and two passageways 7,8 outside of said 
diaphragm. 
The invention is described below with reference to the desalination of sea 
water which, however, constitutes only an example. The invention is not 
restricted to such an application, but can be utilized for each liquid to 
be diaphragm distilled. 
In the following example, furthermore, the first inner pipe is assumed to 
be intended to transport a hot medium. In FIGS. 1,2 and 3 "WW" designates 
warm water, "SW" designates salt water, "PW" designates product water, 
"CW" designates cold water and "AIR" designates air. 
The system operates as follows. Warm water WW is pumped through the first 
passagweay 5. The wall of the first pipe 1 is thereby heated, whereby salt 
water SW pumped through the second passageway 6 is heated. The heated salt 
water emits water vapor which passes through the diaphragm 2 into the 
third passageway 7. The surface of the third pipe 3 is cooled by cold 
water CW, which is pumped through the fourth passageway 8, to a 
temperature below the dew point of the vapor. Vapor having passed through 
the diaphragm condenses against the inside of the third pipe 3, which 
surface thus constitutes the condensation surface, and the condensed fresh 
water PW is conducted away by the third passageway 7 to a collecting tank 
or the like. The product water, thus, is desalinated salt water. 
The structural design of the system implies that it comprises both 
distillation elements and heat exchangers. The system, therefore, is 
simple and compact compared with known systems. 
The way in which the water in the first passageway is heated or the water 
in the fourth passageway possibly is cooled, does not constitute a part of 
the present invention. It is, however, possible to heat water WW by means 
of solar panels and thereafter to pump this water into the first 
passageway 5. The cold water CW can, for example, consist of sea water in 
cases when the salt water SW is sea water. The system then can be arranged 
at or in the sea. 
A heat pump can be utilized for heating and, respectively, cooling water to 
be pumped into the system. 
The system preferably is positioned so that the longitudinal axis of the 
unit is vertical. 
The third passageway 7 preferably is connected at its upper end to the free 
atmosphere or to a suitable gas. 
The reason for this is to prevent a vacuum to be produced in the third 
passageway 7 at the condensation of the vapor. In order to maintain a 
radial distance between the diaphragm 2 and the condensation surface, i.e. 
the inside of the third pipe 3, according to a preferred embodiment a 
distance member is provided in the third passageway 7. Such a distance 
member can be, for example, a net, a folded plastic section or another 
suitable member. 
The first pipe 1 and the third pipe 3 can consist of thin plastic foils. In 
such a case distance members of the aforementioned kind can be provided in 
several or all of the passageways 5,6,7,8. 
The unit described above is particularly suitable for being coupled in 
parallel with several similar units to a greater unit. 
In such a case the fourth, outer pipe 4 is designed to enclose several 
units, each of which comprises said first, second and third pipe 1,2 and 
3. 
In FIGS. 2 and 3 an embodiment of a system is shown where fourteen units 
have been coupled in parallel, and the fourth pipe 4 encloses all units. 
In FIGS. 2 and 3 the same designations have been used for corresponding 
details as in FIG. 1. 
According to a preferred embodiment, shown in FIGS. 2 and 3, each of the 
units is designed so that the pipes have different lengths. The first 
inner pipes 1 are the longest ones and open with their respective ends 
9,10 into a supply and, respectively, removal passageway 11,12. The said 
second pipes, i.e. the diaphragms 2, are shorter and open with their 
respective ends 13,14 into a supply and, respectively, removal passageway 
15,16. 
The third pipes 3 are shorter than said second pipes 2 and open with their 
respective ends 17,18 into a removal passageway 19 for distillate and, 
respectively, into a supply passageway 20 for air. The said fourth pipe 4 
encloses all units and is provided with a supply passageway 24 and a 
removal passageway 25. At the embodiment shown in FIG. 3 warm water WW is 
transported in the first passageway 5 and salt water SW is transported in 
the second passageway 6. In the fourth passageway 8 cold water CW is 
transported. Product water PW is taken out from the third passageway 7 via 
the removal passageway 19. Air is permitted to pass in to the third 
passageway 7 via the supply passageway 20. All of the first, third and, 
respectively, fourth passageways are mutually connected at their lower 
and, respectively, upper ends. 
Each of said passageways 5,6,7,8, thus, are connected to each other by 
having supply and removal passageways in common, so that all units thereby 
are connected in parallel. 
Such a system is very compact and yields a high production of desalinated 
sea water. The structural design of the units permits great variation in 
respect of the number of units, which is of great advantage because of the 
demand of systems having low capacity or high capacity. What has to be 
adapted to the capacity is only the number of units, the size of the 
fourth, outer pipe 4 and the attachment means for the remaining pipes. 
The attachment means may be, for example, three spokes 21,22,23 between 
each of adjacent pipes, as shown in FIG. 2. Such sets of spokes can be 
located at the ends of the pipes and possibly in one or several places 
along the length of the pipes. 
In all Figures the first, second and third pipes 1,2,3 are shown to be 
circular-cylindric. Although this design is preferred, of course, the 
pipes can be given any desired cross-sectional shape, for example square 
shape. 
At times it is desired to bring about turbulene in the passageways in 
excess to that occurring naturally in the passageways. For this purpose, 
means for giving rise to turbulence can be inserted in the passageways, 
for example a net, blades or other details. 
It is obvious that the system can be modified in many ways apparent to the 
expert without abandoning the basic idea of the invention, viz. to arrange 
the different pipes one within the other. 
The present invention, thus, must not be restricted to the embodiments set 
forth above, but can be varied within the scope of the attached claims.