WO2019020605A1

WO2019020605A1 - Unit for desalination of water by mechanical steam compression

Info

Publication number: WO2019020605A1
Application number: PCT/EP2018/070000
Authority: WO
Inventors: Winandy FRANÇOIS-MATHIEU
Original assignee: Industrial Advanced Services Fz, Llc
Priority date: 2017-07-27
Filing date: 2018-07-24
Publication date: 2019-01-31
Also published as: EP3658509A1; BE1024466B1; MA52673A1; MA52673B1

Abstract

The unit comprises a hermetically-sealed casing (1) under vacuum, an evaporator-condenser (8) and at least one compressor (12), wherein the evaporator-condenser (8) is arranged to operate at low temperature, is made of a heat-conducting plastic material, and the exchange surfaces (9) thereof are determined such that the transfer of energy (Wt) of the unit is less than or equal to double the minimum desalination energy (Wm), in order to minimize the total desalination energy (W).

Description

Mechanical water desalination unit

Steam

The field of the invention is the desalination of water, mainly sea water or brackish water, to produce drinking water or demineralized water, by the thermal distillation process by mechanical compression of steam, commonly referred to as "mechanical vapor compression" (CMV). It will be noted that such a desalting process is in competition with the processes of reverse osmosis, freezing, electrodialysis, ion exchange, etc.

The minimum desalination energy, common to all processes, to reach the threshold of production of permeate (osmotic pressure reached) or distillate (reached ebullioscopic gap), being Wm, and the transfer energy, to truly produce the permeate or the distillate, being Wt, energy which is inversely proportional to the exchange surface used in the process, the total energy W of desalination is given by the formula:

Wm + Wt

CE being the total energy efficiency of equipment used for desalination, including pumps or compressors and their engines. With the reverse osmosis process, we can admit:

Wt = Wm

With the mechanical steam compression process, Wt is in a range of 7Wm to 15Wm. Thus, with reverse osmosis, the energy to be consumed is of the order of 2.5 to 5 kWh for 1 m ³ of permeate. With the mechanical compression process of In order to produce 1 m ³ of distillate, it is necessary to consume at least 8 kWh and even up to 18 kWh.

The process by mechanical vapor compression, as it is implemented today, therefore has the disadvantage of high energy consumption. But it still has advantages. It is very stable and comfortable to use.

A conventional steam mechanical desalination unit comprises an overall shell, generally under partial vacuum, in order to reduce the boiling point of the water to be evaporated and condensed, in which there is a water bath, the surface of which ensures evaporation, and a heat exchanger immersed in the bath which ensures condensation. It is often a tube-type or plate-type evaporator-condenser (EC). The EC is made of a conductive heat transfer material, one side of which ensures

evaporation and, on the other side, condensation.

The unit is equipped with a steam transport and compression system and ancillary equipment, including a raw water supply system, a partial vacuum system and non-condensable gas elimination system. distillate extraction system and a concentrate extraction system.

While the units currently built use compressors with efficiencies of less than or equal to about 70%, recent developments now make it possible to consider the use of compressors with significantly higher efficiency.

Whatever the process of desalination considered, Wm is a constant of the salinity of the water. Desalination to separate a saline solution (solution 0 or raw water) into two solutions of different salinity, one superior (solution 1 or concentrate) and the other lower (solution 2, or distillate in the case of processes thermal, or permeate in the case of reverse osmosis), Wm represents the energy which is equivalent to the difference of the energy potentials between the two solutions 1 and 2. From the foregoing, it is understood that Wm corresponds to the energy of a zero production equilibrium desalination system, a function of the difference in salinity between solutions 1 and 2, and that any additional energy addition will generate a start of production of desalinated water.

Wt is the energy, complementary to Wm, to ensure a quantity of desalinated water production. Wt serves to overcome the losses of energy in the different trading systems implemented. Thus, in a reverse osmosis unit, Wt mainly corresponds to the hydraulic pressure losses of the permeate which passes through the membranes and, in a distillation unit, Wt corresponds substantially to the thermal differential necessary for the transfer of the latent condensation energy through of the exchanger.

Wt is therefore directly proportional to the required production flow rate, the resistance of the exchangers (reverse osmosis membrane pressure drop or thermal resistivity of the evapo-condenser material), and inversely proportional to the exchanger surface used. artwork.

The problem at the origin of the invention of the present application is therefore to reduce the electrical energy consumption of a desalination unit, or demineralization, of water by mechanical compression of steam and to make the unit less as attractive as a unit implementing the reverse osmosis process.

To this end, the invention of the present application relates to a unit for desalinating water by thermal distillation by mechanical vapor compression comprising:

- a hermetic envelope under vacuum, with, inside,

a space for receiving water to be desalinated,

an evaporator-condenser in said reception space having evaporation and condensation exchange surfaces, at least one compressor with a motor and means of transport to the evaporative vapor compressor and, of the compressor, to compressed steam to be condensed,

the envelope comprising a salt water supply inlet, means for heating the water to be desalinated and distillate, concentrate and non-condensable gas extraction outlets as well as means for evacuating the water. 'envelope,

characterized by the fact that the evaporator-condenser is arranged to operate at low temperature, is made of a heat conductive plastic and its exchange surfaces are determined so that the transfer energy (Wt) of the unit is less than or equal to twice the minimum desalination energy Wm, in order to minimize the total desalination energy W. The solution of the invention is therefore born with the appearance on the market of i) compressors having excellent yields, allowing low operating temperatures, as well as ii) heat conductive polymers (having a certain percentage of nano-carbons) that can be used because of low temperature operation.

It is the conjunction of all these considerations that justifies the inventive step of the invention.

Previously, given the low efficiency of compressors available on the market, no one had dared to design a unit with excessively large exchange surfaces, aimed at significantly reducing energy consumption. In fact, it would have been necessary, in order to significantly reduce this consumption, to build units with exchange surfaces so excessive that the cost of these units would have been prohibitive.

Thanks to the efficiency of the compressors available today, which can exceed 80% or even 90%, the applicant has dared to significantly increase the exchange surfaces of evaporator-condensers but, thanks to the conductive polymers of heat, at a reasonable cost . The vacuum prevailing inside the hermetic envelope is actually a partial vacuum, which is only intended to reduce the boiling point of the water to be evaporated and condensed.

In the context of the present invention, low temperature is understood to mean a vapor temperature of the order of 20 to 60 ° C.

Preferably, the compressor or the compressors of the unit of the invention have a yield greater than 78% and the exchange surfaces are determined so that the desalination transfer energy is greater than or equal to minimal transfer energy Wtm.

The total minimum desalination energy (Wm + Wtm) is the energy that the compressor needs in order to overcome the solution's ebullioscopic difference to its maximum salinity, ie that of the concentrate. If this low limit Wtm was not respected, the part of the evaporator-condenser, in contact with a solution whose ebullioscopic difference is greater than the gap which defines the limit Wtm, would not be operational and the rate of τ conversion of the unit would not be achieved. Conversion rate refers to the ratio of distillate flow to raw water flow, involving the concept of continuous water renewal. The process of the invention, as well as the reverse osmosis process, uses reservoirs whose water is renewed to avoid an infinite increase in concentration salinity. It should be noted that the ebullioscopic difference is the difference between the boiling point of pure water and the boiling temperature of a saline solution, all other conditions being equal. For a salt water with a salinity of 35 g / l and a pressure equal to atmospheric pressure, this difference is 0.54 ° C. The ebullioscopic difference also varies with the boiling temperature. Thus, under a pressure of 0.1 bar, ie a boiling point of pure water of about 46 ° C., the ebullioscopic difference with a saline solution of salinity of 35 g / l will be about 0, 37 ° C. The invention will be better understood with the aid of the following description of several CMV water desalination units and of the desalination process that is implemented therein, with reference to the drawing in the appendix on which

FIG. 1 is a schematic representation of the preferred embodiment of the unit of the invention and

FIG. 2 is an even more schematic representation of a unit of the invention with several effects.

The diagram of the unit of FIG. 1 is that of a preferred embodiment, but which should not be considered as limiting of the invention. Other embodiments are perfectly conceivable and can therefore advantageously refer to those presented in WO 2015/014840.

With reference therefore to FIG. 1, the unit comprises a hermetic envelope 1 under partial vacuum comprising two compartments 2 and 3. The first compartment 2 comprises an enclosure 4 in which is disposed an exchange coil 5 traversed by non-condensable gases. from the second compartment 3, escaping from the enclosure 4 and the compartment 2, and heating the incoming desalination water from a supply source 6. The water to be desalinated and reheated spring enclosure 4 and compartment 2 by a tubing 7.

The second compartment 3, in which the partial vacuum prevails, provides a space for receiving the water to be desalinated, in which an evaporator-condenser 8 is disposed, which in this case comprises a plurality of evaporation tubes 9 and which form two-by-two Condensation ducts 10 opening into the first compartment 2. The inlet of the condensation tubes of

the evaporator-condenser extend, on the opposite side to the first compartment 2, by a nozzle 11 for receiving a compressor 12 and, in part, an enclosure 13 in which the compressor drive motor 14 and an exchange coil 15. The coil 15, traversed by the water to be desalted as described below, is connected here to a ramp 16 watering the water to be desalinated on the evaporator r-condenser 8, the watering being carried out by apples 17.

The two compartments 2, 3 are separated by a partition 18 through which the condensation ducts 10 pass. The second compartment 3, which is the desalination compartment, has a partition 19 for retaining a water of greater salinity. here a concentrate 20, while the desalinated water, that is to say the distillate 21 from the condensation ducts 10, is retained in the first compartment 2.

Are not shown in the figure, because perfectly familiar to the skilled person, the water supply ing of desalination water of the compartment 2 and the chamber 4, the extraction pumps of the distil 21 and the concentrate 20, nor the means of evacuation of the desalination compartment 3.

It should also be noted here that the unit of the invention could comprise several compressors.

In operation, the flowing water to be desalinated by the you bul ure 7 passes through first two exchangers 22, 23 where it is reheated g ith serpen ^¬ two tins 24, 25 respectively driven by the d 21 and the concentrate istillât 20, the two currents from the two heat exchangers 22, 23 being joined to pass through a third heat exchanger 26 fed by a heating iq uide of a heating ^¬ team 27.

At the outlet of this third exchanger 26, outside, like the first two 22, 23, to the enclosure, the water to be desalinated, through a pipe 28, enters the exchanger 13, inside the enclosure.

In the example shown in FIG. 1, the water to be desalinated thus reaches the evaporator-condenser by a film falling from the watering heads 17 which distribute the water on the exchange surface of all the water. evaporation tubes 9. The nozzle 11 is here shaped to transport evaporated steam tubes 9 to the compressor 12 and the compressed steam to be condensed in the ducts 10.

The compressor 12 is again, here, an axial flow compressor, of excellent performance, in this case, here, greater than 78%. The evaporation-condensation tubes 9 are made of a heat conducting polymer, the unit operating at a low temperature.

The total exchange surface of the tubes 9 is very large and such that the transfer energy Wt of the unit is lower, possibly equal to twice the minimum desalination energy Wm and such that, in the example considered , this transfer energy Wt is between the minimum transfer energy Wtm and the double 2 Wm of the minimum desalination energy.

It will be noted that the heating equipment supplying the exchanger 27 may be an electrical resistance, a heat pump or an exchanger supplied with auxiliary thermal energy by steam or hot water. The heating liquid of this exchanger 27 warms the water to be desalinated which will enter the casing 1 through the tubing 28, whereas in the exchangers 22, 23, the water to be desalinated is heated by recovery of the heat energy. outflows of distillate and concentrate.

The unit shown in the figure comprises evaporation tubes. Another CMV water desalination unit could be equipped, not with tubes, but with plates, in the same material as the tubes.

The equipment for supplying the water desalination unit generally comprises reservoirs whose water is renewed in order to avoid an infinite increase in salinity of the concentrate. This renewal of water characterizes the conversion rate τ of the unit.

At constant raw water salinity, the higher the conversion rate, the higher the average salinity of the concentrate and thus the higher the Wm. Wm therefore depends on the salinity of the raw water and the conversion rate applied to the desalination unit.

Example of calculating the parameters of a CMV water desalination unit Let a unit operate on raw water with a salinity equal to 35 g / l, with a partial vacuum evaporator-condenser operating at a steam temperature of 50 ° C, a conversion rate τ of 33%, here a single compressor with a yield of 90% and that of its motor of 95%, and a definition of the exchange surface such that Wt = 2.Wm.

Wm calculation

τ: 33%

T °: 50 ° C (323 ° K)

Maximum salinity of the concentrate: 35 / (1-0.33) = 52.2g / kg

Average salinity of the concentrate: 43.6g / kg

Concentrate PI vapor pressure: 12,058kPa

Distillate salinity: Og / kg (theoretical approximation)

Vapor pressure of P2 distillate: 12.350kPa

P2 / P1: 1,024

Wm: rtin (P2 / Pl) = 17.85 10 ^"6 kWh / mole or 0,99kWh / m ³ (m ³ of distillate), where In is the natural logarithm.

Wtm calculation

Maximum salinity of the concentrate: 35 / (1-0.33) = 52.2g / kg

Vapor pressure Pc (≠ Pl) of the

concentrate: l, 991kPa

Distillate salinity: Og / kg (theoretical approximation)

Vapor pressure of P2 distillate: 12.350kPa

P2 / Pc: 1.030

Result * 1: RTIn (P2 / Pc) = 22.05 10 ^"6 kWh / mole or 1.23kWh / m ³ (m ³ distillate)

Wtm: Result # l-Wm = 0.24kWh / m ³ Wt calculation

Wt 2.Wm = 1.98kWh / m ³

/ _[k W _h Ton _r

CE 0.9. 0.95 = 0.855

W (Wm + Wt) / CE = 3.49kWh / m ³

Calculation of the exchange surface of the heat exchanger

P3 is the operating pressure of the desalination unit

P3 / P1 e ((Wm + Wt) / (Rt))

T ° steam before the compressor (T 50 ° C

T boiling point (Tevap) 50.48 ° C

T ° condensation (Tcond) 51.45 ° C

dT evaporator-condenser 0.97 ° C

Hec 6kW / (m ² ° C)

Hec is the heat exchange coefficient of the evaporator-condenser, determined empirically and fixed, here, arbitrarily

Latent heat (approximation): 635kWh / Ton

Surface of the exchanger S: 635 / (6, 0.97) = 109, lm ^{2 /} Ton.h

(Ton / h distillate)

According to the same hypotheses as in the example above, by now varying Wt over the interval [Wtm, 2.Wm], the following results are obtained:

Wm

0.99 0.240 0.855 1.439 1.024 1.030 50 50.48 50.60 0.12 6 878

0.99 0.495 0.855 1.737 1.024 1.036 50 50.48 50.72 0.25 6 432

0.99 0.990 0.855 2.316 1.024 1.049 50 50.48 50.97 0.49 6 217

0.99 1,485 0.855 2,895 1,024 1,062 50 50.48 51.21 0.73 6 145

0.99 1,980 0.855 3,474 1,024 1,074 50 50.48 51.45 0.97 6,109

These results demonstrate that the combination of features of the invention is necessary for the design of a CMV desalination unit. which offers competitive energy consumption with reverse osmosis desalination. Indeed, the application of a standard CE efficiency, with 70% for the efficiency of the compressor and 95% for that of its engine, would generate either electrical energy consumption too high to compete with the reverse osmosis process, or surfaces

of such a magnitude that the cost of the unit would also be non-competitive.

Finally, it should be noted that the desalination unit may have several evaporation-condensation effects 31, 32, 33. In this case, and in a manner perfectly known to those skilled in the art, the vapor 34 created on the evaporation surface The first effect 31 is channeled to the condensing surface 36 of the following effect 32 and so on until the last effect 33 where the vapor 37 is then transported and compressed in the compressor 38 again before being recycled to head of the first effect 31.

The respective vapor temperatures in the compartments of the three effects 31, 32, 33 are different and drop by a few degrees from one compartment to another, here with temperatures of 50 ° C., 49 ° C. and 48 ° C. respectively. .

The concentrates and distillates from the three compartments 31, 32, 33 are grouped respectively by two pipes 39, 40. As for the water to be desalinated, it arrives in the compartments of the three effects 31, 32, 33 by a pipe 41 and three groups of apples of watering can 42-44.

Claims

1. Unit for desalinating water by thermal distillation by mechanical vapor compression, comprising:

a hermetic envelope under vacuum (1), with, inside,

a space (3) for receiving water to be desalinated,

an evaporator-condenser (8) in said receiving space (3) having evaporation and condensation exchange surfaces (9),

at least one compressor (12) with a motor (14) and means (11) for conveying to the evaporative steam compressor and, of the compressor, compressed steam to be condensed,

the casing (1) comprising a salt water supply inlet (4), means (27, 4, 22, 23, 13) for heating the water to be desalinated and distillate extraction outlets ( 22), concentrate (23) and non-condensable gases (4) as well as means for evacuation of the envelope,

characterized in that the evaporator-condenser (8) is arranged to operate at low temperature, is made of a heat conductive plastic material and its exchange surfaces (9) are determined so that the transfer energy (Wt ) of the unit is less than or equal to twice the minimum desalination energy Wm, in order to minimize the total desalination energy W.

2. Unit according to claim 1, wherein the water receiving space desalination comprises a first compartment (2) comprising an enclosure (4) for heating the incoming desalination water by non-condensable gases.

3. Unit according to one of claims 1 and 2, wherein the water receiving space desalination comprises a second compartment (3) under vacuum in which is disposed the evaporator-condenser (8) having ducts of condensation (10) opening into the first compartment (2).

4. Unit according to claim 3, wherein there is provided in the second compartment (3), a ramp (16) watering water desalination.

5. Unit according to one of claims 3 and 4, wherein the outlet (22) for extracting the distillate is formed in the first compartment (2) and the outlet (23) extraction of the concentrate in the second compartment (3).

6. Unit according to one of claims 1 to 5, comprising several evaporation-condensation effects (31, 32, 33).

7. Unit according to one of claims 1 to 6, wherein the compressor (12) has a yield greater than 78%.

o

8. Unit according to one of claims 1 to 7, wherein the exchange surfaces (9) are determined so that the transfer energy Wt is equal to or greater than the minimum transfer energy Wtm.