Two-stage, high-conversion water desalination system

Abstract

The invention relates to a two-stage desalination system comprising an intake line (1), a final permeate line (7) and a second reject line (5), wherein the system comprises: a first stage consisting of at least two first pressure pipes (11), each connected to a receiving line (3), a final permeate line (7) and a first reject line (4); and a second stage consisting of at least one second pressure pipe (12) connected to the first reject line (4), a second stage permeate line (2) and a second reject line (5), wherein the receiving line (3) is connected to the second stage permeate line (2) and the intake line (1).

Description

[0001] DOUBLE-STAGE, HIGH-CONVERSION WATER DESALINATION SYSTEM

[0002] OBJECT OF THE INVENTION

[0003] The present invention relates to a two-stage, high-conversion water desalination system, configured using reverse osmosis and / or nanofiltration membranes, and comprising a first stage of high conversion and high salt rejection, and a second stage of high conversion and low salt rejection, where the permeate from the second stage is recirculated at the inlet of the first stage for salinity reduction, thus favoring high conversion in the production of desalinated water and a reduction in the volume of brine produced with low energy consumption.

[0004] BACKGROUND OF THE INVENTION

[0005] Membrane processes for saline water desalting, such as reverse osmosis, can be configured in multiple ways, using various stages or steps to achieve different overall water conversion, energy consumption, and final water quality. A multi-step approach involves treating the permeate obtained from the first tube in a second pressure tube. The permeate will have a lower salinity than the source water, with the goal of improving permeate quality, at the expense of lower conversion and higher energy consumption. A multi-stage approach involves treating the brine reject obtained from the first tube in a second pressure tube. The brine reject will have a higher salinity than the source water, with the goal of increasing overall conversion, at the expense of worsening permeate quality and higher energy consumption.For the latter case, considering input salinities analogous to seawater, there is a limitation in using conventional reverse osmosis elements for seawater due to the high osmotic pressure.

[0006] The need to increase the efficiency of both new and existing dealignment facilities has driven the development of new configurations that allow for higher conversion rates while reducing brine discharge volumes with relatively low energy consumption. At the same time, there is clear industrial interest in developing more efficient brine concentration methods compared to traditional brine treatment methods that involve energy-intensive thermal processes. Aspects related to brine management, such as increasingly restrictive regulations, greater environmental awareness, and the search for economically competitive alternatives, are motivating the industry to focus on developing new processes for brine concentration.Furthermore, the growing interest in the recovery of compounds present in brine, a process known by the Anglo-Saxon term "brine mining," has also highlighted the need to concentrate brines in a more energy-efficient manner.

[0007] Typically, processes for recovering compounds from seawater brine are more efficient at higher concentrations, so preconcentration systems are desirable.

[0008] In the state of the art on high-conversion processes for water with salinity equivalent to seawater, which also involve the concentration of the brine produced, there are two main techniques: one based on membrane technology and the other based on thermal distillation-evaporation systems.

[0009] The systems currently on the market are mostly thermal, with distillation-evaporation techniques being the most efficient, although they require large amounts of energy (15-25 kWh / m3) as well as investment to implement them.

[0010] Membrane-based techniques are generally limited in the treatment of high-salinity streams with total dissolved solids (TDS) levels below 70,000 ppm. These techniques utilize high salt rejection membranes, and the pressure applied to the membrane is directly proportional to the salt content of the stream being treated, resulting in pressurizations above 120 bar. In this regard, reverse osmosis systems capable of operating at very high pressures, close to 120 bar (high-pressure reverse osmosis, or HPRO), have recently appeared on the market. The application of HPRO as a second stage in the treatment of seawater streams allows for the production of permeate quality similar to that of the first stage, with a maximum operating pressure close to 120 bar, thus promoting greater overall conversion.However, the main disadvantage is the requirement for the use of non-standard, very high-pressure equipment (pumping, valves, pressure pipes, etc.), which substantially increases the investment cost compared to conventional osmosis systems, which operate at a maximum pressure of 80 bar.

[0011] The two technologies with the greatest technical similarities with respect to the present invention are listed below:

[0012] First, there is the multi-stage low-salt-rejection reverse osmosis system, which also allows for increased water conversion and brine concentration through membrane processes.

[0013] This configuration aims to concentrate brines generated in a conventional reverse osmosis (RO) process by using multiple low-rejection RO membrane processes and applying more than one stage in series.

[0014] It involves the use of RO membranes with low salt rejection to concentrate brines, thereby decreasing the osmotic difference across the membrane and, therefore, allowing for greater water flow. According to theoretical studies, this technology is capable of concentrating up to 4M NaCl, a concentration suitable for feeding thermal processes. However, it should be noted that it could also be applied to achieve lower concentrations. Therefore, it could be useful for minimal liquid discharge (MLD) or zero liquid discharge (ZLD) schemes.

[0015] However, this system may require second steps for permeate refining and does not consider the recirculation to the head of a partially misaligned stream, which in the present invention is what allows increasing the overall conversion with the use of very high flux and low energy consumption membranes to achieve greater energy efficiency in the conversion and greater ease of design and operation.

[0016] On the other hand, there is the technology known as “Split Partial Single Pass,” or SPSP, which allows for reduced energy consumption in desalination processes using reverse osmosis.

[0017] This technology refers to a pressure tube configuration where the permeate from the two membranes located in the tails, which are of poorer quality, is reinjected into the RO feed, with the aim of reducing feed salinity and resulting in lower energy consumption. This technology does not actually involve brine concentration, but rather the preliminary stage of seawater desalination, but thanks to this innovation, overall energy consumption is lower.

[0018] The main difference between the SPSP system and the present invention is that the SPSP considers the recirculation to the header of part of the permeate from the first stage, without a second stage in the process. In the case of the present invention, the stream recirculated to the header comes from the partially desalted water in the second stage.

[0019] WO2013 / 147702A1 discloses a system for continuous desalting of water having variable salinity, the system comprising: (a) a first reverse osmosis (RO) stage, comprising one or more pressure vessels, configured to receive a saline water feed stream to produce a first reject stream and a first permeate stream, wherein the permeate stream contains less salinity relative to the reject stream; (b) a second RO stage comprising one or more reverse osmosis pressure vessels, said second RO stage configured to receive the first reject stream to thereby produce a second reject stream and a second permeate stream; (c) variable pressurization means located upstream of said RO stages, said variable pressurization means capable of adjusting the pressure of said feed stream;and (d) pressure recovery means configured to recover energy from said second reject stream, said pressure recovery means capable of decreasing the pressure of said feed stream.;

[0020] Document W02006067241 A1 discloses a system for desalinating seawater by means of reverse osmosis comprising a pump responsible for pumping the water to be desalinated towards a feeding device and a reverse osmosis membrane that is fed by the device through an external conduit and that has an outlet conduit for the osmotized water and a conduit for rejecting concentrated brine that returns to the device for feeding back the reverse osmosis membrane mixed with the original stream of water to be desalinated.

[0021] The main difference between the system described in document W02006067241A1 and the present invention is that document W02006067241A1 considers the recirculation at the head of the brine obtained mixed with the source water stream to be desalinated, unlike the present invention which considers the mixture of the permeate from the second stage with lower salinity than the source water. Furthermore, document W02006067241 A1 describes a system with a single stage, unlike the present invention which considers a two-stage system.

[0022] DESCRIPTION OF THE INVENTION

[0023] The present invention describes a two-stage desalination system comprising an inlet conduit, through which water to be desalinated is supplied to the system, a final permeate conduit, through which the final desalinated water of the system is obtained, and a final rejection conduit, through which a fluid with a high saline concentration is obtained as the end of the process carried out in the system.

[0024] The system comprises two stages for water desalination, as described below.

[0025] The first stage is made up of at least two first pressure tubes, each of them connected to a receiving conduit, for the water inlet, to a final permeate conduit, through two partial permeate conduits, through which the desalinated water at the end of the process is obtained, and to a first rejection conduit, for the outlet of water with a higher salt concentration and which will be the feed water for the second stage of the system.

[0026] The second stage is made up of at least a second pressure tube to which the first rejection conduit is connected, for feeding with the high saline concentration water from the first stage, a second stage permeate conduit, from which partially desalinated water is obtained (such as with a saline concentration of approximately half that of the input water to be desalinated) intended to feed the inlet of the first pressure tubes, as will be indicated later, and a final rejection conduit, from which a high saline concentration brine is obtained as the end of the system process.

[0027] Thus, a preferred configuration consists of a specific number of pressure tubes in the second stage, and a maximum of twice that number in the first stage. This is due to the need to maintain minimum water flow rates circulating through the system, since part of the feed water from the first stage is used for recirculation, meaning it is directed to the second stage and then returned to the first stage. It should be understood that when referring to pressure tubes, we are referring to the membranes located within them, and therefore, the number of pressure tubes is related to the size of the membrane surface area.

[0028] In the invention, the water supplied through the inlet conduit does not reach the inlet of the first pressure tubes through the receiving conduits, but is mixed with the water from the second-stage permeate conduit, which provides water with a lower salinity level. It also provides an additional volume of water free of solids, colloids, and microorganisms, reducing the volume of pretreated seawater required for the first stage input, such that the desalination process is more efficient in terms of energy, conversion, and space requirements. In this way, the second-stage permeate conduit is connected to an intermediate point between the inlet conduit and the first-stage pressure tube.

[0029] The pressure tubes incorporate reverse osmosis and / or nanofiltration membranes. These membranes are preferably low salt rejection reverse osmosis membranes in the second stage, where the salt flow through the membranes must be adapted to never exceed 80 bar of pressure, so that the rejection is preferably variable. In the first stage, high-flux, high-rejection, and low-energy membranes are used.

[0030] It should be noted that in conventional RO, the osmotic pressure of the permeate is practically zero due to the high salt rejection, which somewhat limits the flow of water through the membrane in the case of salt-concentrated streams at a pressure equal to or less than 80 bar, which is typically the maximum operating value for reverse osmosis membranes. In the case of the low salt rejection membranes considered here, because a certain amount of salt is allowed to pass through them, the permeate stream has a certain osmotic pressure, which allows the aforementioned limitation to be overcome without exceeding a pressure of 80 bar.

[0031] Also preferably, the desalination system comprises a first pump for feeding the first pressure tubes and a second pump for feeding the second pressure tubes.

[0032] In the previous embodiments, regardless of whether it incorporates pumps, the system can include an isobaric exchanger in two different forms: on the one hand, between the second rejection conduit and the inlet conduit, such that the feed water from the receiving conduits to the first pressure tubes can take advantage of the energy of the high saline concentration of the rejection water; on the other hand, between the second rejection conduit and the first rejection conduit of the first stage before feeding the second pressure tube of the second stage, such that the pressure of the second rejection conduit can be used to increase the pressure of the feed of the second stage.

[0033] The system may also comprise a double-stage isobaric exchanger by which both the water from the inlet conduit and the water from the first rejection conduit can take advantage of the energy of the water from the second rejection conduit.

[0034] It must be considered that the system of the invention is scalable, such that, if the preferred configuration consists of incorporating a maximum of twice as many first pressure tubes as second pressure tubes, it is not limited to incorporating a 2:1 configuration and its multiples, but can be any ratio that meets this condition, such as 3:2, 5:3, and so on, depending on the volume requirements of water to be treated and the selection of the osmosis elements to be used.

[0035] DESCRIPTION OF THE DRAWINGS

[0036] To complement the description being made and in order to help better understand the characteristics of the invention, in accordance with a preferred example of practical implementation thereof, a set of drawings is attached as an integral part of said description, in which the following has been represented for illustrative and non-limiting purposes:

[0037] Figure 1 shows a schematic view of the two-stage water desalination system of the invention.

[0038] Figure 2 shows a schematic view of the two-stage water desalination system of the invention in a second embodiment, where a first isobaric exchanger has been incorporated between the rejection of the first stage and the supply conduit of the first stage.

[0039] Figure 3 shows a schematic view of the two-stage water desalination system of the invention in a third embodiment, where a second isobaric exchanger has been incorporated between the reject and the feed of the second stage only. Figure 4 shows a schematic view of the two-stage water desalination system of the invention in a third embodiment, where a two-stage isobaric exchanger has been incorporated, unifying the two previous embodiments to incorporate a two-stage isobaric exchanger.

[0040] Below is a list of the references used in the figures to facilitate their follow-up:

[0041] 1. Inlet duct.

[0042] 2. Second stage permeate duct.

[0043] 3. Receiving duct.

[0044] 4. First rejection channel.

[0045] 5. Second rejection duct.

[0046] 6. Partial permeate duct.

[0047] 7. Final permeate duct.

[0048] 11. First pressure tubes.

[0049] 12. Second pressure tube.

[0050] 13. First bomb.

[0051] 14. Second bomb.

[0052] 15. First isobaric exchanger.

[0053] 16. Double-stage isobaric exchanger.

[0054] 17. Second isobaric exchanger.

[0055] PREFERRED EMBODIMENT OF THE INVENTION

[0056] The present invention relates to a system for the two-stage desalination of inlet water, configured using reverse osmosis membranes (RO and / or nanofiltration in the same pressure tube or independently), and comprising:

[0057] - a first stage of high conversion, low energy consumption and high salt rejection

[0058] - a second high-conversion, low-salt-rejection stage, where the permeate water generated in the second stage is recirculated back to the inlet of the first stage to reduce the salinity of the inlet water, thereby increasing desalinated water production and reducing the volume of brine produced. This not only results in greater desalinated water production, but also generates a smaller, more concentrated volume of brine (compared to that typically found in conventional RO processes), facilitating the extraction of target compounds present in the concentrated brine from the system's final reject.

[0059] Figure 1 represents a diagram of this situation. It can be seen that the system comprises an inlet conduit (1) through which the water to be desalinated is received, a second rejection conduit (5), through which a fluid with a high saline concentration is obtained, and a final permeate conduit (7), through which the final desalinated water is obtained.

[0060] Each of the two stages consists of pressure tubes (1 1 , 12) connected to an inlet pipe for the water to be desalinated, a rejection pipe for the brine outlet and a permeate pipe for the desalinated water outlet.

[0061] Thus, in the first stage there are first pressure tubes (1 1 ), each of them connected to a receiving conduit (3), for the entry of water, to a first rejection conduit (4) and to a partial permeate conduit (6), the two converging in a final permeate conduit (7) through which the final desalinated water resulting from the entire system process exits.

[0062] In the second stage there is a second pressure tube (12) connected to a first rejection duct (4) of the first stage, to a second stage permeate duct (2) and a second rejection duct (5).

[0063] As can be seen in this figure 1 , the inlet duct (1 ) is directly connected to the second stage permeate duct (2) and subsequently branches into the receiving ducts (3), which are in communication with the corresponding first pressure tubes (1 1 ) located in a first stage of the desalination process. In this way, the water received in the receiving ducts (3) is a mixture of the water that circulates through the inlet duct (1 ) and through the second stage permeate duct (2).

[0064] In these first pressure tubes (1 1 ) are connected the partial permeate duct (6), which subsequently join to form the final permeate duct (7), and the first rejection duct (4), which also communicates directly with the second pressure tube (12) of the second stage of the system, where the second rejection duct (5) and the second stage permeate duct (2) are also connected. In this way, as indicated at the beginning, the entry of water into the system occurs through an inlet duct (1 ), while the exit occurs through a second rejection duct (5), for the concentrated brine, and a final permeate duct (7), for the desalinated water.

[0065] The number of first pressure tubes (11) in the first stage is greater than the number of second pressure tubes (12) in the second stage. This is due to the need to maintain the feed water flow rates in each of the stages, since part of the feed water in the first stage is for refeed, i.e. it is directed to the second stage and then returns to the first stage.

[0066] In the desalination process of the system of the invention, the fact that the water circulating through the second stage permeate conduit (2) has lost salinity in the desalination process of the second stage implies that the receiving conduits (3) have a lower salinity concentration than the inlet conduit (1), so the desalination process to be carried out in the first stage will be more effective than if this step were not carried out, both in terms of energy and production, due to the use of very high flux membranes given the lower salinity achieved.

[0067] In the first stage there are two first pressure tubes (11 ) in which a misalignment of the water entering through the feed conduit occurs, which is the inlet conduit (1 ) that branches into the corresponding receiving conduits (3) that connect with the respective first pressure tubes (1 1 ). To each of the first pressure tubes

[0068] (1 1 ) is also connected, on the one hand, to a first rejection conduit (4), where a fluid with a higher saline concentration is obtained, and which is connected to the inlet of the second pressure tube (12) located in the second stage of the system. On the other hand, each of the first pressure tubes (1 1 ) is also connected to a partial permeate conduit (6) which join in the final permeate conduit (7) through which the desalinated water at the end of the process is obtained.

[0069] In the second stage there is a second pressure tube (12) in which a new process of misalignment of the water that enters through the inlet duct occurs, which in this case is the first rejection duct (4) of the first stage. To the second pressure tube

[0070] (12) is also connected, on the one hand, to a second rejection conduit (5), where the high saline concentration fluid is obtained from the end of the system process, and a second stage permeate conduit (2) through which desalinated water is obtained in order to feed back the first pressure tubes (1 1 ) of the first stage. For this purpose, the second stage permeate conduit (2) is connected to the inlet conduit (1 ).

[0071] The membranes of the pressure tubes (1 1, 12) are preferably reverse osmosis and / or nanofiltration membranes, and can use membranes with different salt rejections (from 50 to 99.95% salt rejection).

[0072] In addition, the system can also incorporate a pumping system in each of the stages, minimizing the energy consumption of the feed pump during operation and balancing the flows of each of the stages. In this way, a first pump (13) supports the feeding of the first pressure tubes (11) and a second pump (14) increases the pressure of the second pressure tubes (12).

[0073] In a second embodiment, as shown in Figure 2, the system incorporates a first isobaric exchanger (15) that is used to recover the energy from the current of the second rejection conduit (5) to pressurize the inlet conduit (1), reducing the energy input required for the first pump (13) to feed the first stage of the system.

[0074] In a third embodiment, as shown in Figure 3, a second isobaric exchanger (17) is used to recover energy from the stream of the second rejection conduit (5) of the second stage to pressurize the stream of the first rejection conduit (4) of the first stage before feeding the second pressure tube (12) of the second stage.

[0075] In a fourth embodiment, as shown in Figure 4, the system incorporates a double-stage isobaric exchanger (16) with the objective that the water current from the second rejection conduit (5) of the second stage, in addition to exchanging energy with the inlet conduit (1), as indicated in the previous embodiment, also does so with the current from the first rejection conduit (4) of the first stage before feeding the second pressure tube (12) of the second stage.

Claims

1. Two-stage water desalination system comprising an inlet conduit (1) intended to receive the water to be desalined, a final permeate conduit (7) and a second rejection conduit (5), the system being characterized in that it comprises: - a first stage, consisting of at least two first pressure tubes (1 1 ), each of them connected to a receiving duct (3), to a partial permeate duct (6) and to a first rejection duct (4), and - a second stage consisting of at least a second pressure tube (12) connected to the first rejection duct (4), to a second stage permeate duct (2) and to a second rejection duct (5), where: - the receiving ducts (3) are connected to the inlet duct (1), - the partial permeate ducts (6) are connected to a final permeate duct (7), and - the second pressure pipe (12) is connected to the inlet duct (1).

2. The two-stage desalination system of claim 1, comprising a first pump (13) for feeding the first pressure tubes (11) and a second pump (14) for feeding the second pressure tubes (12).

3. The two-stage desalination system of claim 1 or 2, comprising a first isobaric exchanger (15) between the second rejection conduit (5) and the inlet conduit (1).

4. The two-stage desalination system of claim 1 or 2, comprising a two-stage isobaric exchanger (16) for exchanging energy between the second rejection duct (5), on the one hand, and the inlet duct (1) and the first rejection duct (4), on the other hand.

5. The two-stage desalination system of claim 1, wherein the second stage houses low salt rejection membranes, this rejection being variable.

6. The two-stage desalination system of claim 5, wherein the passage of salts through the membranes is adjusted so that the maximum pressure is 80 bar.

Images