WO2004014802A1

WO2004014802A1 - Water-desalting plant

Info

Publication number: WO2004014802A1
Application number: PCT/IL2003/000650
Authority: WO
Inventors: David Taran
Original assignee: David Taran
Priority date: 2002-08-07
Filing date: 2003-08-07
Publication date: 2004-02-19
Also published as: AU2003249550A1

Abstract

The proposed 'water-desalinating plant' (WDP) relates to the low pressure flash evaporation type of WDP, it is designed for desalination of seawater and other natural water sources, for generating drinking water, desalted water for power stations, industrial plants as well as recycling sewage water. The invention is also designed for extracting salt from salt solutions. Proposed WDP is intended to reach following aims: to reduce the cost of water-desalinating process using the method of low-pressure evaporation; to ensure controllability of the desalting process for wide range of temperatures of salt water while maintaining specific energy spending within preset parameters; to increase the output of water-desalinating plant; to ensure high quality of desalination; to increase periods between technical overhauls; to maximize the use of vapor condensation energy to intensify the process of boiling up, in order to lower power-consuming; to reduce size and weight of WDP as well as cost of exploitation to the minimum and also to reduce maintenance personal to a minimum. In the proposed WDP vapor condensation with creation of desalted water takes place as result of heat exchange of vapor with colder salt containing sediment (SCS), immediately after their separation in the same operating volume without energy loses and with option of heat recovery. Namely by way of cyclic evaporation of SCS accompanied by partial replacement of water, intended for desalination. In the proposed WDP the stated above aims are achieved by means of using cyclone for separating water, intended for desalination, into water vapor and SCS. Proposed WDP is an Automatic Control System the supports optimal energetic parameters for wide range of temperatures of seawater and supports the functioning of WDP in most geographical zones that are characterized by the lack of sweet water.

Description

WATER-DESALTING PLANT

BACKGROUND - FIELD OF INVENTION

The invention relates to the low-pressure flash evaporation type of water- desalting plants (WDP), it is designed for desalination of seawater and other natural water sources, for generating drinking water, desalted water for power stations, industrial plants as well as recycling sewage water. The invention is also designed for extracting salt from salt solutions.

BACKGROUND - DESCRIPTION OF PRIOR ART

One of the basic requirements for WDP of this type is high effectiveness.

A device of this kind is known from "Low pressure desalinization device", patent number US 5,064,505. The device contains means for separating seawater into vapor and into salt-containing sediment (SCS). The device in a shape of cylindrical tower has a zone for separation and a zone for discharge of SCS. The device also contains means of intake for water intended for desalination, means for creating rarefaction, means for vapor condensation into desalted water. In this device the desalting process is taking place in law pressure chamber that is directly connected to the saltwater source.

In all devices of this type the means for vapor condensation is a heat exchanger that is placed outside the case of the cylindrical tower and vapor evaporation is taking place from the water surface intended for desalination. As a result extra-cold SCS, that is left after evaporation, is mixing with fresh intake water while sinking, cooling it at the same time. In this case the boiling layer of water has lower temperature that drastically lowers device productivity in other words - efficiency.

The device has to have considerable height and huge size in order to ensure qualitative gravity separation of vapor and SCS that makes it economically nonviable. Aside from the fact that, pressure of saturated vapor depends on water temperature, the productivity of WDP device is dependent on season of the year and is not adjustable.

Another device of this kind is known from "Vacuum distillation system", patent number US 5,049,240. The device contains means (in the shape of cylindrical vacuum distillation column) for separating seawater into vapor and into SCS. The device also contains means for intake of water intended for desalination and means for discharge of SCS, means for creating rarefaction, and means for vapor condensation into desalted water. This device consists of vacuum distillation column, having a column inlet, a bottom outlet and a top outlet. In this device the intake of heated salt water is taking place through column inlet in vacuum distillation column. As a result of which instant boiling and transformation of salt solution into double phase state of water vapor and SCS are taking place.

As a result of gravitational separation, vapor partly containing salt sediment is extracted through the top outlet, followed by condensation, while SCS is extracted through the bottom outlet.

In order to ensure qualitative gravitational separation the distillation column has to have considerable height and huge size and weight. Condensation of vapor that was created during low-pressure evaporation process, of heated salt water is energetically expensive and leads to high cost of desalted water.

At that the process of heating up salt water ineffective and leads to additional rise in cost of desalted water. As stated in the text of patent US5, 049,240, salt concentration in the SCS is 9 times higher than its original concentration in the source salt water. It causes intensive deposition of salt on the surfaces of the device. This process requires frequent stoppage and clean up of the WDP with all the organizational and economical consequences.

The invention is based on the task to create WDP in which the device of vapor condensation will be placed in such a way and will be connected to other elements of WDP so that controllability of the desalting process will be ensured for wide range of temperatures of salt water while preserving specific energy spending within preset parameters. At that the task is to maximize the use of energy of vapor condensation for intensification of salt water boiling process. All that without using any additional, economically nonviable, processes - such as preheating salt water, and by means of that to increase economical effectiveness of proposed WDP.

SUMMARY

Proposed invention is intended to reach the following aims:

- to reduce the cost of water-desalinating process using the method of low- pressure evaporation;

- to ensure controllability of the desalting process for wide range of temperatures of salt water while maintaining specific energy spending within preset parameters;

- to increase the output of water-desalinating plant;

- to ensure high quality of desalination;

- to increase periods between technical overhauls;

- to maximize the use of vapor condensation energy to intensify the process of boiling up, in order to lower power-consuming;

- to reduce size, weight and cost of exploitation to the minimum as well as to reduce maintenance personal to a minimum. In the proposed WDP described above and other tasks are solved in such a way that WDP contains device for separating water, intended for desalination, into water vapor and SCS, by means of low-pressure water evaporation method. Device for separating water contains evaporation zone and zone for accumulation of SCS. WDP contains inlet means for supply of water, intended for desalination, from the source of said water into evaporation zone. WDP contains outlet means for water vapor discharge and for creating rarefaction sufficient for boiling up of water intended for desalination in evaporation zone; this outlet means for water vapor discharge and for creating rarefaction contains outlet branch pipe. WDP contains means for vapor condensation with creation of desalted water; at that means for vapor condensation is a heat exchanger connected to said outlet branch pipe and is placed in the zone for accumulation of SCS. The outlet means for discharge of SCS from the zone for accumulation of said sediment contains discharge pipe.

To ensure controllability of the desalting process, outlet means for discharge of said SCS contains flow regulator, at that there is a level detector of SCS that is connected to said flow regulator in such a way that during the operation of water-desalting plant the level of SCS is sustained within preset limits.

In the proposed WDP vapor condensation with creation of desalted water take place as a result of heat exchange of vapor with colder SCS immediately after their separation in the same operating volume without energy loses and with option of heat recovery. Namely by way of cyclic evaporation of SCS accompanied by partial replacement of water, intended for desalination.

It is practical to use as a device for separating water, intended for desalination, into water vapor and SCS, gas cyclone containing means for creating rarefaction sufficient for boiling up of water intended for desalination in operation zone of cyclone. The use of cyclone enables to increase water evaporation surface and to use vortex separation in it, which leads to additional intensification and better quality of vapor separation process.

Turbulent movement of the vortex in the cyclone leads to dispersion of water jet into water droplets tiny diameter, that in turn causes increase in saturated vapor pressure above their surface, intensification of evaporation, further decrease of droplets diameter, further intensification of evaporation and so on up to complete evaporation. The described above process is an additional effect derived from using a cyclone as a device for separating water, intended for desalination, into water vapor and SCS. At that direction of salt water injection and vapor pumping, as well as the shape of the cyclone, are defined by supporting the maximization of this effect.

Substantial differences of the processes described above that are taking place in the claimed WATER-DESALTING PLANT, by comparison to known devices, do in author's opinion indicate invention step, originality and novelty of this invention. DRAWING FIGURES

The essence of invention is illustrated by means of the following drawing.

Fig.1 - schematically illustrates water-desalinating plant according to the invention.

Fig.2, a, b, c - graphically illustrates optimal control parameters for adjusting the work of WDP and corresponding energy spending during stationary work regime of the compressor and production of 1 tonne vapor as a function of salt water temperature.

Fig.2, a - salt water input per 1 tonne desalted water.

Fig.2, b - power input per 1 tonne/sec desalted water.

Fig.2, c - cost per 1 tonne desalted water.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 schematically illustrates WDP 1 as a device for separating water, intended for desalination, into water vapor 2 and SCS 3, by means of low- pressure water evaporation method. Said device contains evaporation zone 4 and zone for accumulation of SCS 5. Water 6, intended for desalination, from the source of said water 7, is supplied into evaporation zone 4 by inlet means for supply of said water. Inlet means for supply of said water contains: pump 8, pipe 9 and injector 10.

WDP contains outlet means for water vapor discharge and for creating rarefaction sufficient for boiling up of water intended for desalination in evaporation zone 4 that contains: outlet branch pipe 11 and compressor 12, connected to means for vapor condensation that is heat exchanger 13 connected to said outlet branch pipe 11 (on fig.1 - through compressor 12) and is placed in the zone for accumulation of SCS 5. From evaporation zone 4, SCS is withdrawn by means of discharge pipe 14 that contains SCS flow regulator - butterfly valve 15 connected to a level detector 16 of SCS in such a way that during the operation of water-desalting plant the level of SCS is sustained within preset limits. This ensures the optimal functioning of heat exchanger 13.

The WDP as a devicel for separating water, intended for desalination, into water vapor 2 and salt containing sediment 3 can function as a gas cyclone. By cyclone we mean any device, corresponding to the existing state of the art, in which vortex separation of source substance into basic components takes place. In this particular case on Fig.1 , in order for device 1 to function as a cyclone, the injector 10 is the vortex (or swirl) generator, or vortex evaporator, that provides output of salt-water jet at high angular speed of axial rotation. In a different design (not shown graphically) injector 10 can have an asymmetrical side exit into evaporation zone 4, which ensures cyclone functioning according to a classical scheme.

In case when large salt water volume transportation and extraction of SCS is energetically expensive and/or when it is practical to use condensation energy and water heating as a result of compressor work, WDP 1 has an intermediate reservoir 17 placed between the source of water 7, intended for desalination, and inlet means 8, 9, 10 for said water supply into evaporation zone.

The intermediate reservoir 17 is connected to the zone for accumulation of SCS 5 by means of said discharge pipe 14 and with the source of water, intended for desalination by means of inlet 18 and outlet 20 pipes by means of controlled pumps 19 and 21 correspondingly. The means for removal of desalted water contains pipe 22 and controlled pump 23. There is also means 24 for regulating the correlation of mass flow of water through said pipes and through said inlet means for supply of water, intended for desalination (all relations are not shown graphically).

The intermediate reservoir 17 can be a lower component part of said zone for accumulation of SCS 5.

As a butterfly valve 15, any existing state of the art element can be used, that will ensure flow regulation of SCS through discharge pipe 14. The presence of such elements as injector 10 is not obligatory. The existence of said elements leads to increase of productivity of WDP, improves the quality of desalination, prevents boiling up of water outside the operational zone of the cyclone and also prevents the clogging of inlet means of cyclone. By virtue of the cyclone design features, they can contain any known means for self-cleaning of inner surfaces (not shown) that can increase periods between technical overhauls. Evaporation zone 4 and zone for accumulation of SCS 5 can function separately, that may require separate access for more effective and speedier maintenance of WDP - not shown.

The proposed WDP is also an Automatic Control System for all processes that are incorporated into it.

The proposed WDP functions as follows (Fig.1).

Salt water 6 is pumped in by means of controlled water pump 8 through inlet pipe 9 into the injector 10, that is a vortex generator placed at the exit end of pipe 9, and injects salt water jet at high angular speed of axial rotation into the operational zone 4 of cyclone 1. In the operational zone of cyclone 1 rarefaction, sufficient for boiling up of water intended for desalination, is created by means of compressor 12. It provides separation of salt-water phase into vapor and SCS. The process of water evaporation goes on continuously as long as the drops of salt water are present in the operating zone of cyclone. This process terminates when rarefaction inside the operation zone of cyclone is equal to the pressure of saturated vapor at temperature of the SCS drops.

SCS consists of drops of concentrated salt solution and salt particles. In the operational zone of the cyclone 1 a vortex is created (shown by two spiral arrows) made out of vapor molecules and components of the SCS. At that components of the SCS 3 (as heavier ones) are moved to the periphery of the cyclone, they reach its inner side surface and sliding down it into the zone 5 for accumulation of SCS. Water vapor 2 as less heavier is moved to the center of cyclone and is sucked into means for water vapor discharge - outlet branch pipe 11. Vapor 2 from outlet branch pipe 11 pumped by means of compressor 12 into means for vapor condensation that is a heat exchanger 13. Within the heat exchanger 13 vapor 2 transmits to SCS 3 part of its internal energy. As a result of that vapor 2 is condensed and transformed into desalted water 25 which by means of pump 23 is pumped into pipe 22 of desalted water. In order to maintain constant pressure in condensation zone of condenser 13, behind compressor 12 an additional compressor may be placed for pumping out part of the vapor and gases that were dissolved in the salt water before separation - not shown.

At the beginning of the process SCS 3 is accumulated in zone 5 for SCS accumulation. After exceeding the preset level, level detector 16 sends a discharge command to SCS flow regulator - butterfly valve 15, to discharge from zone 5 through discharge pipe 14 the heated part of SCS. This ensures the maintenance of the of SCS level within preset limits which provides the optimal functioning of heat exchanger - condenser 13. SCS 3 from discharge pipe 14 either discharges out of the WDP for further utilization or discharges into intermediate reservoir 17. In intermediate reservoir 17 SCS 3 is mixed with salt water 6. Part of the created solution is channeled into outlet pipe 20 by means of controlled pump 21. Simultaneously with this process salt water from the source is pumped into inlet pipe 18 by means of controlled pump 19 into the intermediate reservoir 17.

Directional movement of water, intended for desalination 6, SCS 3, vapor 2 and desalted water 25 during WDP 1 functioning is shown by corresponding arrows. Means 24 acts as an automatic control system of the WDP. Means 24 contains sensors for registration of process parameters such as flow meters, level meters, pressure meters, thermometers - not shown. As a result of analyzing the data from the above mentioned sensors means 24 controls the work of pumps 8, 19, 21, 23 and compressor 12. In this way the work of WDP within preset parameters is ensured.

The work of the control system 24 is based on the following model of processes taking place within the proposed WDP.

One of the results of compressor 12 works is lowering of temperature of SCS as well as of separated vapor relative to source salt water. At that vapor temperature is higher than SCS temperature. Therefore it is energetically more viable to condensate vapor in heat contact with SCS. At that energy of vapor condensation and compressor work is transferred to SCS - energy recovery takes place. After that the SCS, which has higher concentration than in source salt water can be reused, without preheating, in cyclic separation. Theoretical calculation shows, that salt concentration can be enhanced approximately up to 3 times the start value. At that there will be no additional salt deposits on the structural surfaces of the WDP and there will be no significant change in temperature and the evaporation process.

In some cases changes in salt concentrations within the WDP ranges of temperature, pressures, and salt concentrations, are beyond the limits of accurate energy parameters calculation. In those cases salt concentration can be enhanced up to the levels where no sizable salt deposits on the structural surfaces of the WDP.

In order to ensure dynamic stability of WDP parameters, the correlation between the mass of desalted water Δm, disposed through pipe 22, and intake of salt water ml, through inlet pipe 18, and SCS mass m2, disposed through pipe 20, should follow the formula (1):

Δm = ml - m2. (1)

At that salt concentration in the intermediate reservoir 17 grows m1/m2 times. Mass mo of water, pumped through pipe 9, that corresponds to generation of mass Δm of desalted water, can exceed ml + m2 by tens of times. It means cheap transportation of salt water even when the water source is far away. In further calculations those expenses are not taken into account. Therefore, one of the functions of automatic control system 24, is dynamic support of correlation (1), by means of coordinating the output of pumps 19, 21 and 23. The ratio between ml and m2 can be increased. As a result, concentration of salt in SCS will grow by the same proportion. Concentrated SCS may be directed to further salt extraction.

Graphics, represented on Fig.2, a, b, c are based on the following theoretically acquired equations of processes (2)-(8) in the proposed WDP.

1. Temperature at which SCS goes into heat exchanger immediately after separation

2. Vapor temperature r _ 1 0 ./ 1 steam (3)

3. Mass mo of salt water 6 that has to be pumped inside of evaporation zone 4 in order to receive vapor of mass Δm

4. energy spending E_pump for pumping Π\Q kg of salt water equals to energy spending for raising same water on to height h meters:

^Epump ^a m₀ x g h. (5)

5. Power W_pump of pump at yield = k_pump , where 0<k_pump <1, equals:

Wpump ^s m₀ x g x h / k_pump (6)

6. Overestimated compressor work equals:

J-J comp

7. Power W_comp of compressor at yield = k_comp , where 0<k_comp <1 , equals:

^com ⁼ ^comp ' ^comp ^■ W

In equations (2)-(8) the following symbols are used:

Δm=1000kg - vapor mass that is condensed in time unit; mo - mass mQ of salt water 6 that has to be pumped out from evaporation zone 4 in time unit; TQ - start temperature at which salt water 6 is injected into evaporation zone 4; P«l - pressure that is created by compressor 12 in evaporation zone; T**! - temperature of SCS that is in a state of equilibrium with its vapor at pressure P**j ;

^steam " ^vaP^or temperature in evaporation zone 4; atm 1^"boil =373K - boiling point of water at atmospheric pressure; P =10 n/m - normal atmospheric pressure; 5 q=24.8x10 J/kg=

=44640 J/moie - evaporation heat at 273K; γ=C_p /C_v =1.32 - adiabatic constant for water vapor;

3 c=4.18χ10 J/( K°χkg) - specific heat of water;

R=8.31 J/( K°χmole) - universal gas constant;

-3 μ=18χ10 J/( kg/mole) - molar mass of water.

Fig.2a graphically illustrates the correlation of specific salt-water input mo per

1 tonne desalted water (or vapor) that corresponds to equation (4) between process parameters. One of the functions of control means 16 and 24 is dynamical support of equation (4) as a function of salt-water temperature and by means of coordinating the output of pumps 8, 23 and butterfly valve 15.

Energetically the efficiency of WDP work can be represented as summary energetic spending on transportation and pumping in of salt water, compressor work as well as extraction of desalted water and SCS.

Fig.2b graphically illustrates the correlation of specific power of pump 8 and compressor 12 as well as their combined power, required for producing 1 tonne of desalted water per second, from temperature of salt water and when ^kcomp ⁼kpum ^ss°-⁸- These graphics correspond to equation (6) and (8). One of the functions of control means 24 is dynamical support of equations (6) and (8) as a function of salt-water temperature and by means of coordinating the output of pumps 8, 23 and compressor 12. Subject to the additional effect derived from using a cyclone, equations (6) and (8) can be considered as an upper bound of true energy spending that is overstated by several times.

Fig.2c graphically illustrates the correlation of specific cost of production 1 tonne of desalted water per second, from temperature of salt water that takes place at the proposed WDP. This correlation has a clear minimum of 5.5 kwh while the temperature is 20-21 °C. Acceptable maximum of specific cost at 7 kwh corresponds to sea water temperature in the range from 15°C to 32°C. It means that the use of the proposed WDP is economically cost-effective along most seacoasts on the planet where there is a lack of sweet water.

Fig.2 a-c illustrates possibility and necessity of functioning WDP as an Automatic Control System that supports optimal energetic parameters for wide range of temperatures of sea-water and possibility of functioning of WDP in most geographical zones that are characterized the lack of sweet water.

The changed mathematical model of the evaporation process takes into account the changes in vapor pressure above curved surfaces (in this case - tiny drops of water). Using the new process model, it is possible to write a differential equation of the evaporation process, taking into account vortex separation of water flow into tiny drops in the operation area of the cyclone. The solution of this differential equation is a formula that links the parameters of the process. This formula is the quantitative model of the evaporation process and leads to the following results.

In case of non-vortex evaporation the formula corresponds to tabular data of pressure and temperature of water saturated vapor, with the accuracy within 1-3°C (~1% from the absolute temperature).

In case of vortex evaporation the formula leads to the following advantages by comparison to non-vortex evaporation:

- the evaporation pressure is multiplied by seven (7!) from problematic 0.01 bar to easily reachable 0.07 bar;

- at that the temperature of SCS goes down by 6° and as result vapor output grows;

- vapor evaporation pressure in operational zone is much higher than condensation pressure and does not require energy for compression into condensation zone (vapor collapse takes place in the condenser);

- main energy requirements are for water pumping and functioning of vortex generator. These requirements vary from 2.7 KWH/ton at water temperature of 15°C up to 1.0 KWH/ton at water temperature of 35°C. It has a tendency to decrease with further temperature rise, such as salt water used for turbo-generators cooling and channeled to desalting afterwards.

The energy cost for desalting water, arrived at by means of revised model, corresponds to modern requirements.

In order to sustain radial component of water droplets speed and theirs fragmentation up to 1μιw, the energy required is higher by ~1% from the energy required for water pumping, while at the same time the layer thickness is reduced to necessary value of 1 μwi.

For this purpose we use the model of evaporation zone of claimed plant that contains vortex generator in the center of the round bottom of the dome-type evaporation zone, and exit of pumped vapor is placed in the middle part of the top of said dome.

According to the parameters described below the given model corresponds to the proposed experimental plant with the output from 70 to 200 tons of desalted water in 24 hours.

The radius of dome bottom is 3 meters, the height is 10 meters and the radius of vortex generator is 3 cm. The height of salt-water stream in output of vortex generator is 1 cm.

The water is pumped in under pressure that provides its spouting up to the height of 7 meters. Vertical component of drop speed is:

Vy = J2* g *h « 12— sec

Time of the drop flight is:

(this time is enough for full low-pressure evaporation). Time of reaching the dome walls by the drops is: R_

^~ Vx Or:

As a result we got that Vx=~10% from Vy and for supplying of radial component of drop speed needs

of pumping energy.

What will happen to the water stream height, when it will reach the dome wall?

So, if we consider the water volume to be constant (without taking into consideration volume reduction as a result of evaporation), we can write

V = h₀ xS₀ = h„ x S„ or

Sn k„ = h χ .^ — = \μm .

This example shows that necessary parameters of water desalting plant could be achieved with minimal additional energy.

At that the achieved parameters are sufficient for sustaining the claimed process.

In described above experimental desalting plant for all evaporation process a layer of 1 cm in height was used. By using higher volume for evaporation - in meters of height, plant productivity will be increased by many times.

Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.

Claims

CLAIMES:

1. The water-desalting plant, contains:

- device for separating water, intended for desalination, into water vapor and salt containing sediment, by means of low-pressure water evaporation method;

- inlet means for supply of water, intended for desalination, from the source of said water into evaporation zone;

- outlet means for water vapor discharge and for creating rarefaction sufficient for boiling up of water intended for desalination in evaporation zone;

- said outlet means for water vapor discharge and for creating rarefaction containing outlet branch pipe;

- means for vapor condensation with creation of desalted water;

- said means for vapor condensation that is heat exchanger connected to said outlet branch pipe and is placed in the zone for salt containing sediment accumulation;

- outlet means for discharge of said salt containing sediment from the zone for accumulation of said sediment that contains discharge pipe.

2. The water-desalting plant according to claim 1, wherein outlet means for discharge of said salt containing sediment contains flow regulator, at that there is a level detector of salt containing sediment that is connected to said flow regulator in such a way that during the operation of water-desalting plant the level of salt containing sediment is sustained within preset limits.

3. The water-desalting plant according to claim 1 or 2, wherein device for separating water, intended for desalination, into water vapor and salt containing sediment, is a cyclone.

4. The water-desalting plant according to claim 1, 2 or 3, wherein there is an intermediate reservoir placed between the source of water, intended for desalination, and inlet means for said water supply into evaporation zone; this intermediate reservoir is connected to the zone for accumulation of salt containing sediment by means of said discharge pipe and with the source of water, intended for desalination, by means of inlet and outlet pipes; and there is means for regulating the correlation of mass flow of water through said pipes and through said inlet means for supply of water, intended for desalination.

5. The water-desalting plant according to claim 4, wherein said intermediate reservoir is lower part of said zone for accumulation of salt containing sediment.

6. The water-desalinating plant, according to claims 1 to 5 as described above with reference to the enclosed specification.