WO2006106158A1

WO2006106158A1 - Independent reverse osmosis desalination units which are connected in terms of energy flow

Info

Publication number: WO2006106158A1
Application number: PCT/ES2005/000210
Authority: WO
Inventors: Javier Alday Ansola; Juan José RODRIGUEZ GONZALEZ; Jacinto Curbelo Fernandez
Original assignee: Empresa Mixta De Aguas De Las Palmas S.A.
Priority date: 2005-04-05
Filing date: 2005-04-21
Publication date: 2006-10-12
Also published as: ES2277509B1; ES2277509A1

Abstract

The invention relates to independent reverse osmosis desalination units which are connected in terms of energy flow and which use the brine produced in each desalination unit in order to drive the feed flow to the following desalination unit through the use of energy recovery systems. The feed fluid is driven through a pump to the first desalination unit, such as to produce desalinated water and brine which is conveyed to the first energy recovery system, said system being equipped with a feed water inlet conduit and an outlet conduit for conveying the water that has been driven through the recovery system to the second desalination unit. Said second unit produces desalinated water and brine which is conveyed to the second energy recovery system in order to drive the feed water to a pump which raises the pressure to the same pressure as the first pump for feeding the first desalination unit.

Description

CONNECTED INDEPENDENT REVERSE OSMOSIS DESALINATORS

ENERGETICALLY

Indication of the Technical sector

The present invention falls within the field of water desalination by the reverse osmosis method.

State of the prior art.

Desalination of water by the reverse osmosis method is a process where, from a water with a certain content of salts, two flows of water are obtained, one with a low content of salts and the other with a high content of salts.

In this process, semi-permeable membranes are used, which allow the passage of water and do not allow the passage of salts or allow it in a small proportion.

In order for the water to pass through the semipermeable membrane, it is necessary for the feed water to be desalinated to be at a pressure higher than its osmotic pressure, for which reason high pressure pumps are used to drive this water towards the membrane process.

The solution with a high salt content, waste, is at high pressure, for which reason desalination plants usually have systems for recovering this energy, which is transferred or delivered to the feed stream.

Three water flows are involved in a water desalination process: one inlet or supply and two outlet. Of these two outlets, one of them has a lower salt concentration than that contained in the feed; this is called desalinated water or product. The other outflow has a concentration of salts higher than that contained in the feed, which is why it is called concentrate, brine or rejection. In the event that the desalination was carried out '^ b ^ the reverse osmosis process, the product is also called permeate. This name comes from the fact that the desalination, in this case, is produced by means of semi-permeable membranes, in which the water that passes through or permeates through the membrane is the product, with a low content of salts, while the flow of feed, which does not cross the membrane, is left with the salts that do not pass through it, increasing its concentration, which will come out of the process as concentrate, brine or rejection.

Commercial membranes are used for desalination by reverse osmosis, the most common external physical form of which is cylindrical, centrally crossed by the permeate tube. The membranes used in most commercial desalination facilities are manufactured with useful surfaces between 300 and 400 square feet of semi-permeable membrane, the cylinder having dimensions of 8 inches in diameter and 40 inches in length (1 meter) .

These membranes are placed inside a cylindrical container, resistant to the high pressures at which the process is carried out, whose internal diameter adjusts to that of the membrane. This container is called a pressure tube or box.

The pressure tube is provided at both ends with caps, also resistant to working pressure.

The caps have both the connections to the external pipes and to the membranes. Although sometimes, some external connections start from the ends of the pressure tube and not from the caps.

The connections between the covers and the membranes are made by means of tubular pieces provided with O-rings that are coupled to the central tube of permeate of the membrane.

Likewise, two and more membranes can be coupled, through their central permeate tubes, by means of watertight tubular pieces. Pressure tubes are manufactured with the capacity to contain from 1 single membrane to 8 membranes coupled to each other.

The production capacity of a membrane is directly related to its useful surface (300 to 400 square feet). Putting more membranes in the same tube, it is possible to produce more permeate water, without increasing the feed flow.

It is defined as a conversion factor to the quotient between the flow rates of desalinated water and feed water. By the aforementioned, it can be understood that using pressure tubes with capacity for more membranes, a higher conversion factor can be achieved; a higher yield of feed water would be obtained.

In summary, as the number of membranes within a pressure tube increases, the conversion factor increases, achieving greater production from a certain amount of feed water.

Apart from the available surface, the desalinated water production capacity of a membrane is a direct function of the difference between the hydraulic pressure of the fluid and its osmotic pressure.

For this reason, in a pressure tube with several membranes, the hydraulic pressure of the fluid is maximum at the entrance to the pressure tube and decreases due to friction and because part of the water is diverted to the permeate. However, the osmotic pressure of this same fluid is minimal at the inlet and increases because it is concentrated in the salts left by the water that has permeated through the membrane. Consequently, the maximum productivity is achieved at the beginning of the pressure tube, to decrease as the fluid moves through it, that is, the first membrane always has a production per unit area greater than the rest.

For hydraulic reasons, the amount of water that can be introduced into a pressure tube is limited, so the feed water is limited. If you want to treat a quantity greater than this limit, several equal pressure pipes are connected in parallel by means of both supply and concentrate and product collectors.

This whole set of pressure pipes connected in parallel by collectors is called a stage.

Just as the feed water that can be introduced into a pressure tube is limited to a maximum flow, the flow of concentrated water is limited to a minimum, because this, in addition to having a higher content of salts, has to drag and take the suspended solids that entered with the feed water.

Therefore, along the pressure tube, as permeated water is produced, the flow decreases on the feed-reject side, and it is necessary to design so that the concentrate flow never becomes so low that it passes turbulent to laminar type; unless the feedwater is free of suspended solids. The lower limit of concentrate flow per pressure tube will depend on the suspended solids content of the feed water. This not only depends on the quality of the raw water but on the quality of the pre-treatment of the desalination plant.

Explanation of the invention.

Due to the aforementioned, it is because of this that there are no pressure tubes with a greater capacity than for 8 membranes.

However, when with one stage, the concentrated water is still capable of producing more desalinated water, due to its salt content, it is necessary to add another stage, consisting of other pressure tubes connected in parallel by collectors.

In other words, to increase the conversion factor of a plant, apart from the solution of inserting a greater number of membranes per pressure tube, there is a solution to configure the plant in several stages. That is, the rejection of one stage would be the feeding of another stage, constituting the configuration of two stages.

The number of pressure tubes of the second stage must be less than that of the first, so that also in this stage the concentrate flow is higher than the minimum allowed, without the flow rate per tube exceeding its maximum.

The multi-stage design aims to keep the reject flow high, for this, before it becomes dangerously low, the flow rates of various tubes are joined in a collector and reintroduced into a smaller number of tubes pressure, thus increasing the unit flow.

Thus, as many stages as necessary will be installed until the concentrated water is at the saturation limit of some of the contained salts.

The stages can be connected directly from the concentrate collector of one to the supply of the following or pumps can be interposed that increase the pressure of the supply fluid to overcome both the pressure losses due to friction and the increase in osmotic pressure of the water at as it concentrates on salts.

In the event that pumps are inserted between the stages, so that the hydraulic pressure of the feed fluid is increased above its osmotic pressure in an amount similar to the input of each stage, there will be a better distribution of flows and a better use of membranes and all investment.

In this way a similar unitary production of desalinated water is achieved in all stages, achieving that, for example, in a two-stage configuration, with a

37% of membranes in the second stage, their contribution to production approaching said value, 37%. In practice, for a seawater desalination plant, the above means that the intermediate pump must raise the hydraulic pressure of the fluid by a value of around 17 bar above the pressure it had at the outlet of the previous stage. Otherwise, in the case of two stages without an intermediate pump, the second stage would have a much lower performance than the first, so that containing, for example, about 37% of the installed membranes could contribute with only about 22% to the production of desalinated water from the plant.

On the other hand, the semipermeability of the membranes is not perfect, allowing not only the passage of water but a small passage of salts. When the desalinated water or product of the previous installation has a content not suitable for the use that will be given, because the salt content is higher than desired, it is necessary to subject this desalinated water to a second desalination process, which is called the second step. Therefore, it is called the first step to the previous process, consisting of one or more stages. The second step, in turn, can be made up of several stages.

In this second step, as indicated, the water from the previous step is used as feed water, using a pump to drive it to the process.

As a novelty in this application, a desalination plant can be designed in another way, several hydraulically independent desalination units whose only connection is the use of the energy contained in the concentrate of one of them to drive the feed water from another of them, by an energy recovery system, either by means of a turbo-pump or by the use of any of the positive displacement energy recovery devices by the isobaric chamber system or the like.

Until now, energy recovery devices are used for the desalination unit itself. The novelty is in using the energy recovery devices for another unit, in principle, independent.

Actually, the advantage occurs when positive displacement energy recovery devices are used by the isobaric chamber system or the like, since the efficiency of these devices exceeds 92%. In this way, the use of motorized pumps to drive fluids is minimized, whose combination, in the best of cases, can have a performance of 75%.

Another advantage, with respect to designing a single desalination unit with the total capacity of the whole, is the possibility of being able to modulate production, operating with part of the installation. There may be a need to produce less for various reasons: the demand, breakdowns in a part of the installation, etc.

One way to apply the present invention is to use two independent single-stage desalination units, in such a way that the salty feed water that enters the desalination system is directed in part to the high-pressure pump that sends said liquid to the first desalination system or unit, and part to two energy recovery systems through ducts. The positive displacement energy recovery device may be an isobaric chamber system or the like, with associated valves, although other types of energy recovery systems may be used.

The desalinated water obtained from the first reverse osmosis unit is evacuated while the brine is sent through a conduit to the first energy recovery device. This device will serve to:

• Boost the feed salty water that arrives directly at this device to the second reverse osmosis unit with sufficient pressure to produce desalinated water, the use of supplementary feed pumps not being necessary.

• Evacuate the brine from the first desalination unit.

The desalinated water obtained from the second reverse osmosis unit is evacuated through another conduit while the produced brine is sent through another conduit to the second energy recovery device. The feed water will be sent from this device to the first desalination unit and the brine will be evacuated. This brine is at a pressure lower than that of the feed salt water at the entrance to the first reverse osmosis unit, so the salt water pressure must be slightly increased feed that leaves the recovery device, using a pump to equalize the pressure of the fluids.

Brief description of the content of the drawings

For a better understanding of what is described in the present specification, the figures are attached.

Figure 1 represents a conventional flow diagram of the high pressure unit of a water desalination plant by the reverse osmosis method in a stage with recovery of the energy from the concentrate by means of a turbine.

Figure 2 represents a conventional flow diagram of the high pressure unit of a water desalination plant by the two-stage reverse osmosis method with recovery of the energy from the concentrate by means of a turbine, without a pump between stages. <

Figure 3 represents a conventional flow diagram of the high pressure unit of a water desalination plant by the two-stage reverse osmosis method with recovery of energy from the concentrate by means of a turbine and with a pump between stages

Figure 4 represents a conventional flow diagram of the high pressure unit of a water desalination plant by the reverse osmosis method in one stage with recovery of the energy from the concentrate by means of a positive displacement energy recovery device by the isobaric chambers or the like.

Figure 5 represents the object of the invention, where two desalination plants with a single stage have been represented, as an example, with two positive displacement energy recovery devices by the isobaric chamber system or the like, which take advantage of the energy contained in the concentrate of one of them to drive all or part of the feeding of the other. Figure 6 again represents the object of the invention with the valves necessary to be able to modulate production, operating with part of the installation.

Detailed exposition of an embodiment of the invention.

In figure 1 it is observed how the turbo-motor-pump group (3), (2) and (1) drives the salty feed water supplied by the inlet (26) towards a reverse osmosis unit (6). Desalinated water is obtained from said unit (6), which is evacuated through a desalinated water circuit (30), and the brine, which is sent to the turbine (3) through the brine circuit (28 ) and subsequently evacuated to the drain (27). The turbo-motor-pump group (3), (2) and (1), is composed of the high-pressure pump (1) for supplying salt water to the reverse osmosis unit (6), the electric motor (2 ) actuation of the pump (1) and the energy recovery turbine (3) of the brine and joint actuation of the aforementioned pump (1).

From the high pressure pump (1), in the case that it is of the centrifugal type, the water can pass through a regulating valve (4) to adjust the pressure and flow to the conditions suitable for the unit of Inverse osmosis

(6). In case the high pressure pump (1) is of the positive displacement type (piston), said valve (4) would not exist.

Finally, the valve (5), which in the case of the presence of the turbine (3) would be the shutter of the fluid injection nozzle on the rotor thereof, is the main regulation element for adjusting the pressure and the feed flow to the reverse osmosis unit (6). The system may lack a turbine (3), in which case this valve (5) would be a regulating valve, needle or male type.

Figure 2 represents a system similar to that of Figure 1, with the exception that the desalination plant has two stages of desalination by reverse osmosis, numbered with figures (6) and (7). The desalinated water obtained from the first stage of reverse osmosis (6) is channeled by the circuit (30) while the brine is channeled through a conduit (28) to a second stage reverse osmosis (7) from which the desalinated water is again obtained, which is evacuated through the conduit (31), and a brine, which is directed through the circuit (29) to the motor pump group (3), (2) and (1) to take advantage of its energy in the turbine (3), before being evacuated through the drain (27).

In figure 3 a system similar to that of figure 2 is represented, where an intermediate motor pump (9) and (8) is added to the second stage, to optimize the flows in the system, since the production of permeated water is a function of the difference between the hydraulic pressure of the fluid and its osmotic pressure. To homogenize the flows, this pump (8) must increase the hydraulic pressure of the fluid that enters the second stage in an amount similar to the increase in its osmotic pressure with respect to the supply to the first stage plus the loss of pressure supported in it. . That is, the brine produced by the reverse osmosis unit (6) is directed by the conduit (28) where a pump (8) increases the hydraulic pressure of this brine that enters the reverse osmosis unit (7) by an amount similar to the increase in its osmotic pressure with respect to the supply to the first stage plus the loss of load supported in it. The brine resulting from the second desalination unit by reverse osmosis (7) is directed by the circuit (29) to the turbo motor pump group (3), (2) and (1) to take advantage of its energy in the turbine (3), before being evacuated down the drain (27).

For the start-up of the motor-pump (9) and (8), an anti-return valve (10) is installed in by-pass, so that when the pump (1) starts and before the pump (8) starts, the fluid passes through the valve (10), not infringing the movement of the pump rotor (8), with the consequent risk that its motor (9) becomes a generator. When starting the pump (8), the valve (10) closes due to the increase in pressure downstream.

Figure number 4 shows how part of the feed salt water from the inlet (26) is directed to the high pressure pump (1), and part to an energy recovery system (11) through the duct (32) . The electric motor (2) drives the pump (1) sending part of the water from the inlet (26) to the single stage reverse osmosis unit (6). The other part Ia drives the energy recovery system (11). The desalinated water obtained from the reverse osmosis unit (6) is evacuated by the circuit (30) while the brine (28) is sent to an energy recovery device (11). The positive displacement energy recovery device may be an isobaric chamber system or the like (11), with associated valves (12, 13, 14, and 15). In some of the existing devices, all the valves have been replaced by a single slide with various combinations and in other devices they have been totally eliminated. Likewise, the piston (16) represented within the energy recovery device (11), exists in some of the existing commercial devices, but has been eliminated in most of them. The simplest representation has been made to understand the method of operation.

Finally, the booster pump (17) to aid the energy recovery device and its electric drive motor (18) are observed.

The operation of the energy recovery device (11) is as follows. In the valve (13) there is feed water at the available pressure after having passed the pre-treatment, close to 1.5 bar. Opening the valves (13) and (15) at the same time, the cylinder (11) will be filled with that water, moving the piston (16) to the right, that is, next to the valves (14) and (15) . Thereafter, said valves (13) and (15) are closed and valves (12) and (14) are opened.

The brine or concentrated fluid that leaves the reverse osmosis unit

(6) is at a pressure slightly lower than that of the feed fluid at the entrance to the unit (6). When opening the valves (12) and (14), this concentrated fluid enters the cylinder (11) and pushes the piston (16), compressing the feed water that had previously entered and forcing it to leave through the valve (12) to The same pressure of the concentrated water. At this point, feed water is now available at a pressure slightly lower than that of the feed fluid at the entrance to the unit (6), so that only the pressure of the fluid needs to be slightly raised, by means of the pump ( 17) to match it to the power supply to the reverse osmosis unit (6). Subsequently, a new cycle begins, closing the valves (12) and (14) and opening the valves (13) and (15), and the low-pressure feed water enters by displacing the piston (16) and pushing the concentrated water to exit to the drain (36) at low pressure through the valve (15).

Figure 4 shows the energy recovery device with a single cylinder (11). In reality, commercial devices have at least two cylinders, so that when one of them is admitting low pressure feed water, the other is delivering high pressure feed water.

The object of the invention, represented in Figure 5, shows a case of two independent single stage desalination units (6) and (19). This figure is similar to that represented in figure 4, with the exception that the concentrate from the reverse osmosis unit (6) enters the recovery device (20), which feeds the reverse osmosis unit (19), while that its concentrate is introduced into the recovery device (11), which feeds the reverse osmosis unit (6).

That is, part of the salty feed water from the inlet

(26) is directed to the high pressure pump (1), and part to two energy recovery systems (11) and (20) through the ducts (32) and (34). The electric motor (2) drives the pump (1) sending part of the water from the inlet (26) to the first reverse osmosis unit (6) and the other part is sent by the energy recovery system (11).

The desalinated water obtained from the first reverse osmosis unit (6) is channeled by the circuit (30) while the brine is sent through a conduit (28) to the energy recovery device (20). This brine reaches the valve (23).

Opening the valves (22) and (24) at the same time, the cylinder (20) will be filled with the salty feed water coming from the inlet (26), moving the piston (25) to the right, that is, to the side of the valves (23) and (24), evacuating through the valve (24) the brine that was inside the cylinder (20). Thereafter said valves (22) and (24) are closed and valves (21) and (23) are opened.

The brine or concentrated fluid that leaves the reverse osmosis unit (6) is at a pressure slightly lower than that of the feed fluid at

The entrance to the unit (6). When opening the valves (21) and (23), this concentrated fluid enters the cylinder (20) and pushes the piston (25), compressing the feed water that had previously entered and forcing it to leave through the valve (21) to The same pressure of the concentrated water. This feed salt water is sent to the second desalination unit by reverse osmosis (19) through the conduit

(35) with the pressure supplied by the recovery system (20), this being sufficient to produce desalinated water in the second reverse osmosis unit

(19), the use of supplementary feeding pumps not being necessary.

Therefore, the second reverse osmosis unit (19) is designed to work with a lower working pressure than the first reverse osmosis unit (6).

The desalinated water obtained from the second reverse osmosis unit (19) is evacuated through the conduit (31) while the produced brine is sent through the conduit (29) to the energy recovery device (11). This brine is at a pressure lower than that of the feed fluid at the entrance to the unit (6). When opening the valves (12) and (14), this concentrated fluid enters the cylinder (11) and pushes the piston (16), compressing the feed water that had previously entered and forcing it to leave through the valve (12) to The same pressure of the concentrated water. This feed water that leaves the device (11) is at a pressure lower than that of the feed fluid at the entrance to the unit (6), for which reason it is led through the duct (33) to the pump (17) to match it to the power supply to the reverse osmosis unit (6). Subsequently, the valves (12) and (14) are closed and the valves (13) and (15) are opened, producing the evacuation of the brine, through the valve (15), starting a new cycle, closing the valves (12) and (14) and opening the valves (13) and (15)

In figure 6 the valves (38) have been added, with positioner for regulation, and (39) to (43), necessary to be able to take out of service one of the reverse osmosis units (6) or (19) and one of the recovery devices energy (11) or (20), the device then remaining similar to that represented in figure 4.

Figure 6 will have the work system explained for Figure 5 with the valves (39) and (42) closed and with the valves (40), (41) and (43) open.

When you want to take the reverse osmosis unit (19) out of service, keeping unit (6) in service, the valves (39), (43) and (41) will be closed and the valves (40) and (42).

In case of stopping the reverse osmosis unit (6), keeping in service

Ia (19), valves (40), (43) and (42) would be closed and Ia (39) and Ia (41) would be opened. In this case, in addition, the feed flow to the reverse osmosis unit (19) must be regulated by means of the regulating valve with positioner (38) because this unit (19) works with a flow rate lower than the unit (6 ).

Although it has not been represented in Figures 5 and 6, these desalination installations may include turbo-motor-pump groups (3), (2) and (1), as Io exposed in Figures 1 to 3 to take advantage of through The turbine (3) the energy of the brine and transmit it to the first high pressure pump (1) for feeding the salt water that drives the reverse osmosis unit (6), or the other pumps or booster (17). In turn, in the event that the independent desalination units have several desalination stages, these may include motor pump systems (8) and (9) with valves (10) as shown in Figure 3.

Claims

1. Desalination installation by the reverse osmosis method, characterized by consisting of several independent desalination units (6 and 19) where the energy contained in the concentrate flow of one of them (6) is used to drive the feed flow of the following unit (19), through the use of energy recovery systems (20) and the concentrate flow of the last unit (19) is used to drive the feed flow of the first unit (6) by using of an energy recovery system (11).

2. Desalination installation by the reverse osmosis method according to claim 1, characterized by the existence of turbines (3) for energy recovery from the evacuation of brine for energy supply to the high pressure fluid supply pump (1) incoming.

3. Desalination installation by the reverse osmosis method according to claim 1, characterized in that the energy recovery devices (11 and 20) used to drive the feed flow of the following unit are positive displacement devices based on isobaric chamber systems or similar.

Desalination installation by the reverse osmosis method according to claim 1, characterized in that the desalination units are made up of one or more desalination stages.

Desalination installation by the reverse osmosis method according to claim 4, characterized in that the desalination units that are composed of several stages have intermediate motorcycle pump devices (8) and (9) with anti-return valves (10) in parallel to The pump.

6. Desalination installation by the reverse osmosis method according to claim 1, 2, 3, 4 and 5, characterized in that it consists of a feed fluid inlet (26), a high pressure pump (1) that drives part of the feed to the first desalination unit by reverse osmosis (6), a duct (30) that collects the desalinated water produced by the unit (6), a duct (28) that collects the brine produced by Ia first unit ^~ desalting (6) and the addresses the first set of energy recovery (20) to which arrives a conduit (34) with part of the feed water from the inlet (26), and leaves other duct (35) that leads the feed water driven by the energy recovery system (20) to the second desalination unit (19), a duct (31) that collects the desalinated water produced by the unit (19) and another one conduit (29) that directs the brine produced by the second desalination unit (19) to the second energy recovery system (11) to which a conduit (32) arrives with part of the feed fluid from the inlet (26) and from which another duct (33) leaves with the feed water driven by the recovery system (11) to a pump (17) that raises the pressure of this feed fluid to the same pressure as that of the pump (1) high pressure to send this fluid to the first desalination unit (6), in conjunction with which the pump (1) sends, and why each energy recovery system (11) and (20) includes a device or conduit for evacuating the brine from the reverse osmosis units (36) and (37 ).

7. Desalination installation by the reverse osmosis method according to claim 6, characterized by applying the same principle of energy recovery set forth in the preceding claim to 3 or more independent desalination units.

8. Desalination installation by the reverse osmosis method according to claims 6 and 7, characterized in that the desalination installation has a set of valves (39 to 42) that allow one or more desalination units to be taken out of service while the other desalination continue to operate.