Patent Application
20140154099
This invention relates to devices and processes for pumping liquids with energy recovery; to membrane filtration, for example by reverse osmosis; and to desalination.
Many areas of the world do not have adequate fresh water supplies but they are near seawater. Seawater can be desalinated using reverse osmosis (RO). During RO, the feed water must be pressurized above the osmotic pressure of the feed water. The feed water becomes concentrated during this process and its osmotic pressure increases. Feed water pressures for seawater reverse osmosis (SWRO) are typically in a range of 50-70 bar (approximately 725 psi to 1015 psi).
Pressurizing the seawater in an RO system consumes energy. One approach to reduce energy consumption is to recover energy from the residual pressure of the brine after it leaves an RO module. An energy recovery pumping system is described by Childs et al. in U.S. Pat. No. 6,017,200 entitled “Integrated Pumping and/or Energy Recovery System.” This approach uses multiple water cylinders moving in a phased relationship to provide pressurized feed water to a RO membrane unit. One side of a piston in the water cylinder drives the feed water to the RO membrane unit while the other side of the piston receives brine from the RO membrane unit. The pressure of the brine reduces the power required to move the piston. Each water cylinder is connected to a separate hydraulic pump and hydraulic cylinder combination to move the piston in the water cylinder according to a desired velocity profile and to provide the additional energy required to pressurize the feed water.
U.S. patent application Ser. No. 13/250,463, entitled “Energy Recovery Desalination”, by D'Artenay et al. describes an energy recovery pumping system that makes various improvements to the Childs et al. system. For example, each of the hydraulic pumps has an adjustable swash plate to change the rate and direction of hydraulic fluid flow to its associated hydraulic cylinder. Inner and outer control loops are used to modify the position of the swash plate so that the water cylinder connected to the hydraulic cylinder follows an intended velocity profile more closely.
A liquid pumping system is described in this specification that comprises a plurality of liquid pumps and a hydraulic drive unit. Each liquid pump is driven by a separate hydraulic cylinder. The hydraulic cylinders are powered by a shared hydraulic pump through a valve set. The valve set is operated by a valve set controller. The valve set controller is configured to distribute a flow of hydraulic fluid from the hydraulic pump between the hydraulic cylinders such that the liquid pumps operate in a phased relationship to each other. Preferably, the total liquid flow produced from the liquid pumps is generally constant over a period of time in which the hydraulic pump produces a generally constant output. Optionally, the valve set may comprise a set of valves, for example a proportional directional control valve for each hydraulic cylinder, connected in parallel to the hydraulic pump.
A membrane filtration system is described in this specification that uses the liquid pumping system to provide feed water to a membrane unit. A water circuit is configured such that each liquid pump receives pressurized brine from the membrane unit while pumping water. The membrane unit may be a reverse osmosis unit.
Processes are described in this specification for pumping a liquid and for treating water. The liquid pumping process comprises a step of providing an initial flow of pressurized hydraulic fluid. The initial flow of pressurized hydraulic fluid is distributed between a plurality of hydraulic cylinders such that, over a period of time in which the initial flow is essentially constant, the sum of the distributed flows is also essentially constant but the hydraulic cylinders move in a phased relationship to each other. Each hydraulic cylinder drives a liquid pump. In the water treating process, water is pumped to a membrane unit. Brine from the membrane unit is provided to each liquid pump while that liquid pump is feeding water to the membrane unit. Preferably, the liquid pumps produce a generally constant flow of feed water to the membrane unit. Optionally, the membrane unit may be a reverse osmosis unit.
The processes and systems provide useful alternative ways and means for pumping liquids or treating water. In at least some cases, the processes and systems may provide one or more benefits relative to the systems described by Childs et al. and D'Artenay et al., or other high pressure pumping systems with energy recovery, such as reduced energy consumption, reduced parts count or cost, or reduced maintenance. Without limitation, the processes and systems may be used in the desalination industry.
The pumping system 11 has two or more water cylinders 14 and a hydraulic drive unit 18. The water cylinders 14, and the valves and conduits of a water circuit connecting them to the membrane unit 16, may be similar to those described in U.S. Pat. No. 6,017,200, entitled “Integrated Pumping and/or Energy Recovery System”, U.S. patent application Ser. No. 13/250,463 entitled “Energy Recovery Desalination” and U.S. patent application Ser. No. 13/250,674 entitled “Valve System for Pressure Recovery in IPER”, which are incorporated herein by reference. The pumping system 11 shown has three water cylinders 14 but alternatively there may be two, three, four or other numbers of water cylinders 14. Alternatively, other types of water pumps may be used in place of the water cylinders 14. The pumping system 11 may also be used to pump other liquids.
Feed water, for example seawater, brackish water, groundwater, boiler feed water or wastewater, flows from the feed water source 12 to the water cylinders 14 via low pressure feed pipes 20. The feed water is pressurized within the water cylinders 14 and directed to the membrane unit 16 via high pressure feed pipes 22. Each water cylinder 14 goes through approximately the same cycle but the cycles have a phased relationship to each other such that at any given point in time each water cylinder 14 is in a different part of its cycle.
The membrane unit 16 separates the feed water into a low pressure stream of low-solute permeate and a high pressure stream of high-solute brine, alternatively called concentrate or retentate. The permeate is withdrawn from the membrane unit 16 for various uses, for example drinking water, through permeate pipe 23. The brine is directed back to the water cylinders 14, via high pressure brine pipes 24. Each water cylinder 14 receives brine while providing feed water such that the pressure of the brine can be used to help pressurize the feed water. Low-pressure brine, after being used to help generate feed water pressure, is directed from the water cylinder 14 for waste, recycling or reuse via low pressure brine pipes 25. The water cylinders 14 are dual acting pumps that pump feed water on both a forward and a reverse stroke.
Variations of the system 10 may have two, three or more single acting or dual acting water cylinders 14. The description immediately below will focus on one water cylinder 14 and the movement of a single reciprocating assembly 26 that is part of the water cylinder 14. However, other parts of the description and figures may refer to a particular water cylinder 14, 141, 1411 or to a set of the water cylinders 14, 141, 1411.
Referring to
The water pistons 32 are mechanically coupled to each other by a connecting rod 38. The connecting rod 38 extends through a dividing wall between the concentrate working chambers 36, 36A and out of the water cylinder 14 through bearing and seal assemblies (not shown), which minimize or prevent pressure or fluid leaks. The connecting rod 38 and the dual-acting pistons 32 are collectively referred to as the reciprocating assembly 26.
The reciprocating assembly 26 is connected to a piston rod 40 of a hydraulic piston 52 (see
Each water cylinder 14 comprises water cylinder valves 70 that control the flow of liquid into and out of the water cylinders 14. Opening and closing of the water cylinder valves 70 is controlled by a controller 90 in association with the movement of the reciprocating assembly 26. Optionally, the water cylinder valves 70 may be similar to those described in U.S. Pat. No. 6,017,200, entitled “Integrated Pumping and/or Energy Recovery System”, U.S. patent application Ser. No. 13/250,463 entitled “Energy Recovery Desalination” and U.S. patent application Ser. No. 13/250,674 entitled “Valve System for Pressure Recovery in IPER”.
While the reciprocating assembly 26 moves forwards, or upwards as it is oriented in
The hydraulic piston 52 optionally includes an extension 41 that extends from the first side 56 of the hydraulic piston 52. The extension 41 preferably has a different cross-sectional area than the piston rod 40. In particular, the extension 41 may have a smaller cross-sectional area than the piston rod 40. The first side 56 and the second side 60 of the hydraulic piston 52 preferably have different surface areas. In particular, the first side 56 of the hydraulic piston 52 preferably has a larger surface area than the second side 60. For example, the ratio of the surface areas of the first and second side 56, 60 may be within about 10% of the ratio of the forces acting on the water pistons 32 as they move in the forward and reverse directions within the water cylinder 14. The ratio of forces is calculated by equation (1) below and it is equal to the piston surface area within the first feed water working chamber 34 (PSA 34) subtracted by the piston surface area within the first concentrate working chamber 36 (PSA 36) relative to the piston surface area within the second feed water working chamber 34A (PSA 34A) subtracted by the piston surface area within the second concentrate working chamber 36A (PSA 36A):
Ratio of forces=(PSA 34−PSA 36):(PSA 34A−PSA 36A) (1).
The ratio of forces can be within a range of about 1:1 to about 1.25:1. The ratio of hydraulic piston 52 surface areas is selected to help balance a pressure differential that arises between the hydraulic cylinders 44, 44′, 44″ resulting from the connecting rod 38 extending through the second feed water working chamber 34A but not the first feed water working chamber 34. Alternatively, but not preferably, the connecting rod 38 may be extended through the first feed water working chamber 34.
If the extension 41 is not included, optionally the piston rod 40 may be re-sized to ensure that the ratio of hydraulic piston 52 surface areas is still within 10% of the ratio of forces acting on the water pistons 32. If this results in piston rod 40 being too small to withstand the hydraulic forces, the surface areas of the water pistons 32 can be modified to ensure that a sufficiently large piston rod 40 diameter is used while simultaneously providing the desired ratio of the hydraulic piston 52 surface areas.
Over a period of time, for example an hour or more, when a generally constant flow of feed water to the membrane unit 16 is desired, the hydraulic pump 42 is operated at a generally constant output. The hydraulic pump 42 provides a generally constant flow of hydraulic fluid at a generally constant pressure through supply pipes 50 to the valve set 45. The hydraulic pump 42 may be one of a number of variable displacement pumps, including but not limited to: axial piston pumps, bent axis pumps and pressure compensated variable displacement pumps. Alternatively, the hydraulic pump 42 may be one of a number of fixed displacement pumps, including but not limited to: rotary vane pumps, piston pumps and diaphragm pumps, with a motor that may be controlled by a variable frequency drive unit. Return pipes 51 conduct hydraulic fluid returning from the valve set 45 to a hydraulic fluid reservoir 49. Optionally, a filter may be provided in the return pipes 51.
Optionally, the hydraulic pump 42 may supply hydraulic fluid to the valve set 45 through an accumulator 48 to accommodate temporary pressure increases or decreases in the supply pipes 50. Optionally, the hydraulic drive unit 18 may further comprise a pressure relief loop 96 with a pressure relief valve 97. The pressure relief loop 96 connects the supply line 50 to the hydraulic fluid reservoir 49. The pressure relief valve 97 opens if pressure in the supply line 50 exceeds a pre-set pressure indicating a failure in the hydraulic drive unit 18 or the pumping system 11.
For each hydraulic cylinder 44, a forward feed pipe 54 connects the valve set 45 to a chamber of the hydraulic cylinder 44 in communication with the first side 56 of the hydraulic piston 52. A reverse feed pipe 58 connects the valves set 45 to another chamber of the hydraulic cylinder 44 in communication with and a second side 60 of the hydraulic piston 52.
The valve set 45 receives a generally constant flow of hydraulic fluid from the hydraulic pump 42 and distributes the hydraulic fluid between the hydraulic cylinders 44. For example, in relation to each hydraulic cylinder 44, the valve set 45 may direct pressurized hydraulic fluid to the first side 56 of the hydraulic piston 52 or to the second side 60 of the hydraulic piston 52, or the valves set 45 may stop the flow of hydraulic fluid to the hydraulic cylinder 44. The valve set 45 may also return hydraulic fluid from the hydraulic cylinder 44 to the hydraulic fluid reservoir 49. The valve set 45 may be configured such that low pressure returning hydraulic fluid flows through the same, or a different, valve body that the pressurized hydraulic fluid flows through.
Optionally, the valve set 45 may comprise a separate control valve 46 for each hydraulic cylinder 44. The control valve 46 may be, for example, one or more servo valves, preferably with actuator feedback. Alternatively, the control valve 46 may be one or more four-way, proportional directional control valves. Table 1 below provides a summary of some of the available positions of a four-way, proportional directional control valve 46. Each control valve 46 is able to transition between position 1 and position 2 and between position 2 and position 3. The individual control valves 46 in the valve set 45 are operated in a phased relationship to each other. However, operation of the control valves 46 is coordinated such that the sum of the flow rates of pressurized hydraulic fluid to forward feed pipes 54 and reverse feed pipes 58, which is essentially the same as the sum of the flow rates in the individual supply pipes 50′, 50″ and 50′″, is essentially constant over a period of time in which the flow rate in the supply pipe 50 is essentially constant. The operation of the control valves 46 can also be coordinated to minimize pressure losses across each control valve 46. This is achieved by directing a position profile of each control valve 46 to closely follow the shape of a velocity profile 138 of the associated water piston 32, as described further below. In Table 1, positions 1 and 3 represent nominal fully open positions. However, these positions may be partially open, for example 80% to 98% open, positions in the physical valves to allow for a controller to correct errors by temporarily more fully opening a valve, as will be described further below.
Preferably, the control valve 46 has a controllable transition speed between the three positions. Preferably, the rate of flow through the valve while transitioning or at a certain time is a known or determinable function of the location of control valve 46 between positions. Optionally, a throttle valve may be integrated with the control valve 46 to vary the flow rate through the control valve in position 1 or position 3. However, it is typically more energy efficient to control flow rate in position 1 or position 3 by varying the output of the hydraulic pump 42.
The spool 64 is moved by the actuator 66. The spool 64 may move by sliding or rotating. The actuator 66 may be a mechanical actuator, a pilot-valve system, an electronic servo system or a combination of devices. The actuator 66 is connected to the controller 90 and moves the control valve 46 when instructed by the controller 90. Preferably, the actuator 66 includes an internal controller 67 that receives the instructions from the controller 90 and instructs the actuator 66 to move the control valve 46. The actuator 66 can move the spool 64 at a predetermined rate of speed. However, it is preferable for the controller 90 to control both the timing and rate of moving the spool 64. Varying the position of the spool 64 alters the velocity of the hydraulic cylinder 44. Varying the rate of movement of the spool 64 alters the acceleration or deceleration of the hydraulic cylinder 44. The control valve 46 preferably includes a spool position transducer 65, for example a linear variable differential transformer (LVDT), which feeds into a control loop within the internal controller 67 so that the position of the spool 64 can be adjusted if required to better match the position instructed by the controller 90 at a particular time. The rate of movement of the spool 64 may be implemented as a series of changes of position over time rather than as a rate directly.
The controller 90 preferably includes one or more programmable devices such as a processor or microprocessor, computer, Field Programmable Gate Array, or programmable logic controller (PLC). Alternatively or additionally, the controller 90 may comprise one or more non-programmable control elements, such as a timer or pneumatic or electric circuit, capable of implementing a sequence of operations. The controller 90 is preferably the same controller that is used to control the water cylinder valves 70 and the hydraulic pump 42. Optionally, multiple controllers may be used, preferably connected to a master controller.
The cycle is implemented by the controller 90 moving the one or more valves of the valve set 45. For example, the controller 90 may have a velocity reference chart that represents the velocity profile 138, or a related position reference chart giving the desired position of the reciprocating assemblies 26 over time, or both. The velocity reference chart is pre-calculated and stored in the memory of the controller 90. The controller 90 is programmed to poll the velocity reference chart, for example at regular time intervals, to determine the required velocity at that time. At each time interval, the controller 90 instructs the valve set 45 to move one or more control valves 46, or hold one or more control valves 46 in position, as required. Accelerations and decelerations are caused by moving a control valve 46 between positions from one time interval to another. The required spool positions and changes in positions over time are obtained by the controller 90 referencing the velocity reference chart and sending instructions to the control valves 46 to move the spools 64 to positions of the spools 64 predicted, according to a chart or formula in the memory of the controller 90 relating reciprocating assembly 26 velocity to spool 64 position, to give the velocity of the reciprocating assemblies 26 specified for that time interval.
In greater detail, and as depicted in
The controller 90 receives information regarding the position of each reciprocating assembly 26 (308). For example, the controller 90 receives the positional information from an assembly position transducer 63 (shown in
The controller 90 instructs the valve set 45 to implement the three velocity profiles 138, 138′ and 138″ in the phased relationship. Where the valve set 45 comprises three control valves 46, each control valve 46 moves through the same cycle but at different times. For example, at time A in
During the velocity profile 138, there are four generally distinct pressures that occur within the system 10. The first pressure P1 is the pressure that supplies the feed water from the source 12 to the water cylinder 14. P1 can be provided by a variety of known pumps. The second pressure P2, which is higher than P1, is the pressure exerted on the feed water from the water cylinder 14 to the membrane unit 16. P2 is provided by the movement of the reciprocating assembly 26. The third pressure P3 is the pressure of the concentrate as it leaves the membrane unit 16 to return to the water cylinder 14. P3 is less than P2 since some of the energy is used to drive a filtration process of the membrane unit 16. The fourth pressure P4 is the pressure of the concentrate as it leaves the water cylinder 14 to the waste or recycling stream. P4 is less than P3. For example, P1 may be in the range of 5 to 100 p.s.i.; P2 may be in the range of 600 to 1000 p.s.i.; P3 may be in the range of 500 to 950 p.s.i.; and P4 may be in the range of 1 to 50 p.s.i.
Preferably, the controller 90 includes an independent proportional, integral and derivative (PID) loop for each control valve 46 (see
To allow for a PID output command indicating a more fully open valve position at any time, the position of each spool 64 may be between 80 and 98% open when the associated reciprocated assembly 26 is at maximum velocity specified in the velocity profile 138. This may be achieved by multiplying the master command by a further correction factor 306. This permits the spool 64 to move to a more open position and increase the flow of hydraulic fluid to the hydraulic cylinder 44 to correct the position or velocity of the reciprocating assembly 26 even if the reciprocating assembly 26 is already moving at the maximum velocity specified in the velocity profile 138.
Optionally, the positional information from the assembly position transducer 63 may be used to modify the output of the hydraulic pump 42. The positional information is received by the controller 90 and the positional information is mathematically transformed into a calculated change in hydraulic fluid flow rate through the control valve 46 that will be required to correct an error in position. The calculated change in hydraulic fluid flow rate is then multiplied by a proportional gain to provide a hydraulic command signal. The hydraulic command signal is sent to the hydraulic pump 42 to cause the pump to vary its output. For example, when the hydraulic pump 42 is a fixed displacement pump that is regulated by a variable frequency drive, the hydraulic command signal is sent to the variable frequency drive to change the hydraulic output. As another example, the hydraulic pump 42 can be an open circuit, pressure-compensated variable frequency pump with an internal control loop. The internal control loop includes a pump controller and a pressure sensor. The hydraulic command signal modifies a pressure threshold set-value within the pump controller so that when the pressure sensor senses an error between the actual pressure and the pressure threshold, the controller can change the hydraulic output to better match the pressure within the pump to the pressure threshold value. This altered hydraulic output also contributes to having the reciprocating assemblies 26, 26′, 26″ in the correct position and at the correct velocity during the cycle.
Preferably, the internal controller 67 receives spool positional information from the spool position transducer 65, which is compared with the instructed position provided by the last master command received. Any error between the spool positional information and the instructed position provides an error signal that is multiplied by a proportional gain to provide a new command signal. The new command signal is sent to the actuator 66 to move the spool 64 to, or closer to, the instructed position. The derivative and integral of the error signal can also be multiplied by individual gains and added to the new command signal to the actuator 66.
The system 10, with three water cylinders 14, 14′, 14″ and three, 4-way, 3-position proportional directional dual pilot control valves 46, was simulated with MATLAB/Simulink software.
Based upon computer simulations of the water treatment system, the system 10 has approximately 7% less specific power consumption than a system using three hydraulic pumps (one for each water cylinder) with swash plates. In the simulation, a significant portion of this difference was attributed to not idling hydraulic pumps during the dwell periods and the lack of an auxiliary parasitic charge pump that is used with swash-plate piston pumps.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.
