NO20151169A1

NO20151169A1 - A system for treating a liquid medium by reverse osmosis

Info

Publication number: NO20151169A1
Application number: NO20151169A
Authority: NO
Inventors: Julia Navarsete
Original assignee: Qrrnt As
Priority date: 2015-09-11
Filing date: 2015-09-11
Publication date: 2017-03-13

Description

INTRODUCTION

The present invention concerns a system for treating a liquid medium by reverse osmosis in a semi-continuous or continuous cyclic process, the liquid medium being a solution, a dispersion or an emulsion or a combination thereof..

In many parts of the world there is a lack of sufficient fresh-water resources and with an increasing population this represents a serious and growing problem. The majority of countries with coastline to the ocean lacks sufficient drinking and fresh-water resources, but the oceans contain 97 % of all water on the planet, and thus great efforts have been made to cover the need for fresh water for drinking and irrigation in agriculture by using desalination units for converting saline water or sea water to fresh water. Present-day desalination units require extremely large amounts of energy as they are based on methods for desalination which are very energy-consuming. This is particularly the case of multistage flash distillation units.

Since the 1990ies desalination units based on reverse osmosis for converting sea water to fresh water have increasingly come into use. Desalination units based on reverse osmosis require less energy than units based on conventional distillation methods, but the reverse osmosis itself demands a very high pressure in the flltration stage.

REVERSE OSMOSIS

Reverse osmosis is a membrane-based flltration method capable of removing many types of large molecules and ions from a solution by applying pressure to the solution on the upstream side of a selective membrane. As a re sult the solutes are retained on the pressurized upstream side of the membrane and the pure solvent passes through the membrane to the other side. The selectivity of a reverse osmosis membrane is such that large molecules or ions shall not pass through pores or holes in the membrane, but that a smaller component of the solution, such as the solvent, passes freely therethrough.

In a normal osmosis process the solvent naturally moves from an area of low solute concentration (high potential) through the membrane to a volume of high solute concentration (low potential). The movement of a pure solvent to equalize solid concentration on each side of the osmotic membrane generates an osmotic pressure. When an external pressure is applied to reverse the natural flow of the pure solvent, this is termed reverse osmosis. This process is sirailar to other membrane-technology applications, but there are essential differences between reverse osmosis and flltration. The removal mechanism in membrane flltration is draining or size exclusion and a flltration process is thus theoretically able to allow perfect exclusion of particles regardless of operational parameters such as the upstream pressure and concentration. Reverse osmosis is a diffusive mechanism and the separation efficiency is dependent on solute concentration, pressure and water flow rate.

PRIOR ART

As well known, reverse osmosis is today used extensively for producing drinking water from sea water by removing salt and other substances thereof from the water molecules. A usual prior art scheme for reverse osmosis is shown in fig. 1 where flow directions are indicated by arrows adjacent to the piping. A liquid medium to be treated, in this case regarded as sea water, is conveyed by a high-pressure pump which delivers pressurized sea water to the upstream side of the membrane in a reverse osmosis unit. The high pressure of the concentration generated by the high-pressure pump forces the water molecules through the membrane of the reverse osmosis unit and produces fresh (desalinated) water on the downstream side. The remaining high-pressure concentrate flows from the reverse osmosis unit via a pressure exchanger where concentrate is drained from the concentrate flow and sea water is fed from the intake. The pressure of the concentrate is transferred to the sea water which is conveyed by a circulation pump to the pressurized feed from the high-pressure pump, thus allowing a regeneration of a part of the energy consumed in the process..

A reverse osmosis requires that high pressure is exerted on the upstream side of the membrane and this pressure will usually be between 2 and 17 bar for fresh and brackish water and as high as 40 to 82 bar for sea water. This is because sea water has a natural osmotic pressure of 27 bar which must be overcome. Reverse osmosis has been applied for desalination of sea water and the production of fresh water for many years, but has also increasingly been applied for purifying fresh water for medical, industrial and domestic applications. It should be mentioned that reverse osmosis is eminently capable of moving particles from a liquid medium down to a size of about 0,1 nm. Reverse osmosis can thus be regarded as a hyper-filtration version of membrane flltration. Large-size reverse osmosis units are now in use for desalination of sea water or generally for purification of water in a number of environraents. Reverse osmosis is also applied for cleaning effluent and brackish groundwater which then usually are treated in an ordinary effluent treatment plant before being subjected to reverse osmosis treatment. Reverse osmosis is also used for production of deionized water and in the food-processing industry and even used in the wine industry. A great advantage of the reverse osmosis process is that it does not require thermal energy. This makes it desirable in processes where conventional heat-treatment processes can be expensive and also detrimental for certain heat-sensitive substances, such as may be found in food products.

The efficiency of the reverse osmosis process depends on a number of factors including membrane sizes and membrane pore sizes, the temperature of the liquid medium and the membrane surface area. But the overall determining factor is the pressure applied to the upstream side of the reverse osmosis membrane. The pressure applied depends on the osmotic pressure of the liquid medium, which in case of sea water is around 27 bar. To operate a reverse osmosis unit the pressure applied to the upstream side should be at least twice as high and usually at least 40 bar and upwards to beyond 100 bar is applied. Portable reverse osmosis unit for domestic use are widely used in private households in many parts of the world, but they are usually operating below 40 bar and thus less efficient. But the yield requirement is not critical under such circumstances.

Typically the high-pressure pumps used in desalination of sea water under industrial conditions generate pressures from about 55 to above 80 bar, but require a large amount energy. The efficiency of small domestic portable units is low as they operate at a very low pressure and are usually not able to achieve a yield of more than 15 to 20 % of the water entering the system. Large-scale industrial and public systems based on reverse osmosis have typically a production efficiency about 75 % or even as high as 90 %. They operate continuously apart from a few hours each day spent for maintenance, but yet such large plants are capable of outputting fresh water in amounts of more than 1000 m<3>per unit and day. The main disadvantage with prior art reverse osmosis systems is the high energy costs. As a matter of fact the dominant cost factor is the energy consumption. It is thus desirable to furnish a reverse osmosis unit which lowers the energy consumption significantly, yet contributes a high efficiency.

OBJECTS OF THE INVENTION

A first object of the invention is to provide a system for reverse osmosis capable of reducing the energy consumption by a significant factor and lowering the cost per unit purified liquid produced.

Another object of the invention is to provide a system for reverse osmosis processes which can be applied in a variety of environments and circumstances and is easy to set up and locate and moreover can be scaled according to need with quite simple measures.

The objects of the invention as well as other advantages are achieved with a system according to the introduction of the claim 1.

DRAWING FIGURES

The invention will be better understood from the following description with reference to the appended figures of which

fig. 1 shows a prior art reverse osmosis system using a pressure exchanger, as already discussed in the introduction,

fig. 2 a schematic diagram of a preferred embodiment of the system according to the present invention,

fig. 3 a schematic diagram of another preferred embodiment of the system according to the present invention,

fig. 4 the system in fig. 2 with values for power inputs and flow rates for a daily fresh water output of 5 000 cubic meters, and

fig. 5 the system in figure 2 with values for power inputs and flow rates for a daily fresh water output of 10 000 cubic meters

DESCRIPTION OF THE INVENTION

The system according to the invention shall now be described in greater detail with reference to the appended drawing figures. The system as such will be better understood by a discussion of the flow diagram of the system as shown in fig. 2, which discloses general overview of a preferred embodiment of the system according to the invention.

A rotary engine such as an electric motor is connected to the drive shaft of a high-pressure pump which preferably in one embodiment can be a high-pressure turbine pump. The other end of the drive shaft is connected to the output shaft of a high-speed turbine. The electric motor in a start-up phase powers the high-pressure pump which pumps and pressurizes the liquid medium, e.g. sea water, such that it emerges at an upstream side of the high-pressure pump with a pressure high enough to effect a reverse osmosis of the liquid medium. The pressurized liquid medium is conveyed to the upstream or concentrate chamber in a reverse osmosis unit and at a pressure at least as high as the osmotic pressure of the liquid medium and preferably much higher than that. The reverse osmosis takes place in the reverse osmosis unit by the pressurized liquid being flltered through the reverse osmosis membrane to emerge from a downstream chamber thereof and at atmospheric pressure as a liquid which in case of sea water will be fresh or potable water. The remaining liquid medium in the upstream chamber of the reverse osmosis unit is now a high-pressure concentrate which emerges as an effluent therefrom and is conveyed to the high-speed turbine which connected to the drive shaft will start to power the high-pressure pump, thus adding additional power to that supplied .by the power output of the electric motor. High-pressure concentrate effluent flows through and powers the high-speed turbine and is emptied via a drain to the surroundings and at a pressure substantial equal to the atmospheric pressure.

As the process continues, the power input from the turbine to the pump increases such that increasingly larger volume of pressurized liquid medium is conveyed to the reverse osmosis unit until a stable process level is attained. Thus substantial part of the pressure energy of the liquid medium is regenerated as the high-pressure concentrate will remain at a pressure level only slightly lower than initial pressurized liquid medium because only a fraction of the pressure energy is lost in the membrane process. It should be understood that the drive shaft common to the electric motor the high pressure pump and the turbine can be provided with couplings for controlling the speed of the shaft in the initial process stages until a stable operation is achieved. However, a far better and cheaper solution is to provide a not shown frequency converter for controlling the speed of the electric motor. A substantial part of the energy required by the high-pressure pump for pressurizing the liquid medium is then supplied by the turbine and the efficiency of the power generation will substantially be determined by the filter efficiency, i.e. the proportion between the filtered output and the effluent concentrate. The flltered output of the liquid medium will as stated be at atmospheric pressure while the energy loss in the flltration process amounts to a very small pressure drop in the effluent concentrate. As much as 40 % or more of the energy required for the process can thus be regenerated in this manner from the pressure energy of the effluent concentrate.

In another preferred embodiment of the system according to the invention shown in fig. 2 the turbine pump is replaced by piston pumps and the common output shaft of the turbine and the electric motor is a crankshaft with cranks and connecting rods joined to the piston rods in the piston pumps. The piston pumps can be single acting pumps in which case it is preferable with two or more piston pumps or piston cylinders to achieve a near continuous operation. The embodiment shown in flg. 2 is in this case delivers essentially two-stroke operation achieved by providing the cranks on the crankshaft mutually opposed by 180°. The number of cylinders can be increased at will, e,g.to in four such single-acting cylinders and can be operated in similar fashion to a four-stroke piston engine. Controllable flashback valves must be provided to the pump intakes and outlets in order to control the flow, and for their operation a control system, e.g. using valve-activating servos, will be needed, but is not shown here. Also it is understood that it should be possible to provide a crankshaft with a flywheel in order to achieve a smooth operation of the pumps.

In another version of the preferred embodiment shown in fig. 3 the piston pumps will be double-acting with intake/outlet valves provided at the top and bottom of the piston cylinders and operating in a manner such that the pressurized liquid medium is delivered in a nearly continuous manner. The piping arrangement for providing the delivery of liquid medium to the reverse osmosis unit must then be rearranged in a different configuration from that shown in fig. 3, but this will be easily understood by persons skilled in the art.

Fig. 4 discloses a first example of the energy recovery with a system according to the invention. The high pressure pump is in this case capable of delivering a maximum of 145 l/s or 12 500 m<3>of liquid medium, in this case tåken to be sea water. A 700 kW electric motor initializes the pump in the start-up phase and as turbine becomes increasingly capable of providing power, the system will stabilize at a total power output of 1200 kW, of which 500 kW is provided by the high-speed turbine fed by the pressurized concentrate effluent from the reverse osmosis unit. The output of this version of the system will be as high as 5000 m<3>per 24 hours or 58 l/s, while the concentrate effluent from the reverse osmosis unit provides 7500 m3 per 24 hours to the high-speed turbine which is able to operate at a continuous power level of 500 kW. This means that somewhat more than 40 % of the energy requirement of the process is regenerated from the pressurized concentrate effluent, while the remainder of the energy requirement is provided by the electric motor operating at 700 kW. The net energy cost of the operation is thus limited to that of the power requirement of the electric motor.

In the example disclosed in fig. 5 the output is doubled to 10 000 m<3>potable water per 24 hours and the turbine power output is now increased to 1000 kW when fed by 174 l/s of high-pressurized concentrate effluent. The regeneration efficiency in this case approaches 45 % of the required 2230 kW needed to operate the high-pressure pump. The electric motor contributes 1230 kW, which will be the determining factor for the operating cost the system in this case.

From the above it is seen that the system according to the invention ena be described as a high-volume high- efficiency reverse osmosis system particularly suited for desalination of large amounts of sea water in any location where sea water or salinated water is present, capable of delivering several thousands of cubic meters of potable water a day. It also should be made of note that the proportion between the output flow of fresh water or potable water and the remaining concentrate effluent shall depend on the filter efficiency, and that a generally high-filter efficiency of 40 % and above shall require a correspondingly high pressure imparted to the liquid medium by the high-pressure pump.

As evident from the discussion of the reverse osmosis process in the introduction, the method and system according to the invention is not necessarily limited to desalination of sea water, but can be used to hyperfiltration of dispersions or emulsions, preferably after they have been subjected to prefiltration to remove the larger constituent particles. However, the present invention can also be applied to purify natural water, i.e. natural fresh water in industrial processes and the like where a high purity is required.

It should be mentioned that reverse osmosis produces a highly purified product. As most minerals and the like are removed, the product becomes acidic and some post-treatment may be required, for instance remineralization.

Claims

1. A system for treating a liquid medium by reverse osmosis in a semi-continuous or continuous process, the liquid medium being a solution, a dispersion or an emulsion, or a combination thereof, wherein the system comprises a high-pressure pump at an intake thereof and connected to a reverse osmosis unit, comprising a reverse osmosis membrane, wherein the high-pressure pump pressurizes the liquid medium to a pressure at least as high as an osmotic pressure thereof and delivers said pressurized liquid medium to the reverse osmosis unit at an upstream side of the reverse osmosis membrane, whereby said liquid medium is separated into two components, one of which passes through the reverse osmosis membrane and emerges de-pressurized at a downstream side thereof, purified of solutes, dispersed or emulsified matter depending on an initial composition of said liquid medium, and wherein the system is characterized in thata high-speed turbine is provided and connected to said reverse osmosis unit at the upstream side of the reverse osmosis membrane and adapted to be powered by receiving a second component of said liquid medium, said second component being a high pressure concentrate of said liquid medium, and in that the output shaft of the turbine is connected to said high pressure pump for imparting power thereto.

2. A system according to claim 1, characterized in thatthe high-speed turbine is a Pelton turbine.

3. A system according to claim 1, characterized in thatthe high-pressure pump is a turbine pump and connected directly or indirectly to the output shaft of the turbine.

4. A system according to claim 1, characterized in thatan initializing and supplementary power-unit is an electric motor connected directly or indirectly to the output shaft of the turbine and adapted for the delivering power to the high-pressure pump in a start-up phase and thereafter or supplying a part of the power needed to power the high-pressure pump.

5. A system according to claim 1, characterized in thatthe high-pressure pump is a piston pump, comprising one or more piston cylinders, and in that the output shaft of the turbine is the crankshaft with a nuraber of cranks corresponding to the nuraber of piston cylinders such that a rotary motion of the crankshaft via the cranks and connecting rods drives a piston in each of said one or more piston cylinders in reciprocating motion.

6. A system according to claim 5, characterized in thatthe piston pump comprises one or more single-acting piston cylinders.

7. A system according to claim 5, characterized in thatthe piston pump comprises one or more double-acting piston cylinders.

8. A system according to claim 5 or claim 6, characterized in thatthe piston cylinders are provided with flash-back valves, controlling the inflow of the unpressurized liquid medium and the outflow of the pressurized liquid medium to the reverse osmosis unit.

9. A system according to claim 1,characterized in thatan initializing and supplementary rotary power unit is provided sharing a common drive shaft with the high-speed turbine and the high-pressure pump.

10. The use of the system according to claim 1 for desalination of sea water.

11. The use of the system according to claim 1 for treatment of waste water.

12. The use of the system according to claim 1 for treatment of fresh water for industrial and domestic purposes.