CONTROL METHOD FOR A MEMBRANE FILTER SYSTEM AND MEMBRANE FILTER SYSTEM

Abstract

A control method uses in a membrane filter system operated in iterative filtration cycles, the cycles including a production period and a following flushing. A setting of a crossflow on the entrance side (4) of a membrane (2) in the production period is controlled such that the energy consumption (E) per filtration cycle reaches an optimum. A corresponding membrane filter system is provided.

Claims

1. A control method used in a membrane filter system, the method comprising the steps of: operating the membrane filter system in iterative filtration cycles, said cycles comprising a production period and a following flushing; and controlling a setting of a crossflow at a concentrate outlet of a membrane in the production period is controlled such that the energy consumption per filtration cycle reaches an optimum.

2. A control method according to claim 1, wherein that said optimum is a minimum energy consumption.

3. A control method according to claim 1, wherein the energy consumption is a relative energy consumption per volume of produced permeate or concentrate.

4. A control method according to claim 1, wherein said crossflow is defined as a flow out of a concentrate outlet.

5. A control method according to claim 1, wherein the crossflow is set by adjusting the flow or speed of a crossflow pump, which crossflow pump at least partly recirculates the crossflow.

6. A control method according to claim 1, wherein the membrane filter system comprises a membrane having a pore size smaller than 10 nm.

7. A control of method according to claim 1, wherein the crossflow is defined by a recovery level defining a ratio of permeate flow and a feed flow.

8. A control method according to claim 1, wherein said setting is varied stepwise for different filtration cycles and the energy consumption for the different filtration cycles is compared to find the optimum.

9. A control method according to claim 8, wherein, when stepwise varying the setting, said setting is kept constant for a number of filtration cycles, a trajectory for the energy consumption over time is generated for this number of filtration cycles and the optimum for the energy consumption is found for the obtainable limit crossflow or flow ratio with a gradient of the trajectory below a predefined limit.

10. A control method according to claim 1, wherein beginning with a starting level a crossflow is reduced in an iterative manner after a number of filtration cycles as long as the gradient of the trajectory of the energy consumption remains below the predefined limit.

11. A control method according to claim 1, wherein beginning with a starting level a recovery level of a ratio of permeate flow and feed flow is increased in an iterative manner after a number of filtration cycles as long as a gradient of a trajectory of the energy consumption with respect to time remains below the predefined minimum.

12. A control method according to claim 1, wherein the energy consumption per filtration cycle includes a total energy consumption for production, flushing and cleaning of the filter system.

13. A membrane filter system comprising: at least one membrane; at least flow regulating device; and a control device configured to control the flow regulating device, to set a crossflow at a concentrate outlet of the membrane during a production period of the membrane filter system, wherein said control device is configured such that the crossflow in the production period is controlled such that the energy consumption per filtration cycle reaches an optimum.

14. A membrane filter system according to claim 13, wherein the at least one membrane has a pore size smaller than 10 nm.

15. A membrane filter system according to claim 13, wherein the at least one flow regulating device is a valve or a pump.

16. A membrane filter system according to claim 13, wherein the control device comprises an energy recording means recording the energy consumption of the filter system.

17. A membrane filter system according to claim 13, wherein the control device is configured to control the flow regulating device by a control method comprising the steps of: operating the membrane filter system in iterative filtration cycles, said cycles comprising the production period and a following flushing; and controlling a setting of the crossflow at the concentrate outlet of the at least one membrane in the production period such that the energy consumption per filtration cycle reaches an optimum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In the drawings:

[0032] FIG. 1 is a schematic view showing a membrane filter system according to the invention;

[0033] FIG. 2 is a diagram showing the energy consumption for different crossflows;

[0034] FIG. 3 is a diagram showing the setting of the crossflow;

[0035] FIG. 4 is a diagram showing the energy consumption for different recovery levels; and

[0036] FIG. 5 is a diagram showing the setting of the recovery rate.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0037] Referring to the drawings, the membrane filter system as schematically shown in FIG. 1 comprises at least one membrane 2 having an entrance side 4 and an outlet side 6. The entrance side 4 and the outlet side 6 may be defined by housing containing the at least one membrane 2. The outlet side 6 is the outlet for the filtrate or permeate and connected to a permeate outlet 8. The entrance side 4 is connected to a feed line 10 having a feed pump 12. The feed line 10 preferably opens to the entrance side 4 at a side or edge of the membrane 2. On the other side or edge there is arranged a concentrate or retentate outlet 14. Thus, the flow from the feed line 10 towards the concentrate outlet 14 provides a crossflow c along the entrance side 4 or parallel along the entrance surface of the membrane 2. A part of the liquid passes the membrane 2 towards the outlet side 6 and the permeate outlet 8.

[0038] In this example there is arranged a recirculation line 16 connecting the concentrate outlet 14 and the feed line 10 downwards the feed pump 12. In the recirculation line 16 there is arranged a recirculation or crossflow pump 18. This crossflow pump 18 is recirculating a part c.sub.1 of the crossflow c, i.e. a part of the flow of the concentrate or brine leaving the entrance side through the concentrate outlet 14. Thus, the crossflow pump 18 provides an additional partial crossflow c.sub.1 along the entrance side 4 in addition to the part c.sub.2 of the crossflow produced by the feed pump and flowing out of a concentrate drain 15. Downstream the branch of the recirculation line 16 there is arranged a valve 20 controlling the partial crossflow c.sub.2 defining the concentrate flow through said concentrate drain 15. The valve 20 may adjust the opening degree of the line towards the concentrate drain 15 and may allow to completely close the concentrate drain 15. If the valve 20 completely closes the concentrate drain 15, the crossflow c would be provided via the recirculation line 16 only since the entire feed flow from the feed line 20 would have to pass the membrane 2 towards the permeate outlet 8.

[0039] The filter system comprises a control device 22 connected to the feed pump 12, the crossflow pump 18 and the valve 20 for controlling the pumps 12 and 18 and the valve 20, i.e., to adjust the opening degree of the valve 20 and the speed of the pumps 12 and 18. It must be understood that the control device 22 may control the valve 20 or one of the pumps 12, 18 only, or control the valve 20 and just one of the two pumps 12 and 18. Furthermore, the crossflow pump 18 or the valve 20 may be omitted. Instead of a valve 20 having an adjustable opening degree, for example, there may be provided a fixed flow restriction. However, in all embodiments the control device 22 can control the setting of a crossflow c along the entrance side 4 of the membrane 2, i.e. a concentrate flow at the concentrate outlet 14 by controlling at least one pump 12, 18 and/or at least one valve 20.

[0040] Furthermore, the system may comprise one or more sensors. In this example the feed flow may be detected by or output from the feed pump 12 and the respective information is transferred to the control device 22 via a data connection between the feed pump 12 and the control device 22. However, there may be an additional flow sensor to detect the feed flow. Furthermore, in this example there is shown a flow sensor 24 on the permeate outlet 8 for detecting the volume flow of filtrate or permeate leaving the filter device. The flow sensor 24 is connected to the control device 22 for transmitting data representing the volume flow in the permeate outlet 8 to the control device 22. Depending on the desired setting of the crossflow c the flow sensor 24 may not be necessary and may be omitted. Instead, it would be possible to arrange further flow sensors, for example on the recirculation line 16 and/or the concentrate outlet 14, in particular downwards the branch of the circulation line 16, i.e. for detecting the flow through the concentrate drain 15. Furthermore, it would be possible to detect or provide the recirculation flow c.sub.1 by the control of the crossflow pump 18. The control device 22 may have an interface for communication with external or further devices.

[0041] According to the invention the control device 22 is configured to control a crossflow c or a setting of a crossflow c along the entrance side 4. FIG. 2 is a diagram showing the cumulative energy consumption E over time t. The energy consumption E preferably is a relative energy consumption per volume of permeate flowing through the permeate outlet 8 or concentrate flowing out of the filter device via concentrate drain 15. Whether the volume of the concentrate or the volume of the permeate is regarded, depends on which flow is the desired outlet of the filter device. The energy consumption E comprises the entire energy consumption for a production cycle and a following flushing of the filter with stopping the production. It particularly comprises the energy needed for driving the pumps 12 and 18. FIG. 2 shows the development of the energy consumption E for four different crossflow levels A, B, D und C. The curve A represents nearly no crossflow. In this case there occurs a fouling of the membrane 2 by which the flow resistance increases and, thus, the energy consumption for pressurizing the fluid, i.e. in particular the energy consumption of the feed pump 12 increases quickly. The curve B represents a too little crossflow. Also, with this crossflow which may be provided by the recirculation pump 18 there occurs a fouling of the membrane 2 resulting in an increase of required energy for pressurizing the fluid. The curve C represents the optimum crossflow in this example having the minimum increase in energy consumption over time. This curve has the lowest possible slope of the cumulative energy curve. The curve D shown in FIG. 2 illustrates what happens when the crossflow is too high. In this case the cumulative energy curve is very straight since the fouling develops very slowly. However, the overall rate of energy consumption E is higher than the optimum, since too much energy is used for providing the higher crossflow, for example by driving the recirculation pump 18 with a velocity higher than necessary.

[0042] To find the optimum crossflow the control device 22 carries out a control method for setting the crossflow as explained with reference to FIG. 3. FIG. 3 shows the overall or cumulative energy consumption E over time t similar to FIG. 2. It must be understood that the total energy consumption as shown in FIGS. 2 and 3 may include energy equivalents representing further costs occurring, for example costs for cleaning agents required to remove a fouling from the membrane and/or costs for treatment of brine or concentrate, respectively. By adding such energy equivalents representing further costs it is possible to take these further costs into consideration in addition to the energy consumption. Thus, by this method not only the energy consumption can be optimized, but the total costs per volume of the filter outlet can be optimized.

[0043] In FIG. 3 the lower curve shows the different crossflow levels, for example provided by the recirculation pump 18. Alternatively, or in addition it may be possible to adjust the crossflow by changing the speed of the feed pump 12 and/or adjusting the opening degree of the valve 20. The upper curves in FIG. 3 represent the energy consumption over the time t. Each dot represents the total energy consumption for one filtration cycle (production+flushing). It is preferred to start with a high or excessive setting of the crossflow and to drive the crossflow down in steps of certain duration. This means, the crossflow is kept constant for a few or several filtration cycles, in this case three filtration cycles, each consisting of a production period and a following flushing. By this a cumulative energy consumption curve or trajectory 26 can be assessed for each crossflow level. At the beginning for the highest crossflow level 28 the energy consumption curve 26 is substantially horizontal, i.e., the slope is substantially zero and the energy consumption E is substantially constant over time. For the next reduced crossflow level 30 the energy consumption curve 26 remains substantially horizontal. In the next step the crossflow level is further reduced to a crossflow level 32. The slope of the energy consumption curve 26 slightly increases, i.e., the energy consumption E increases over time between the three filtration cycles indicated by the dots on the curve 26. When further reducing the crossflow level to a crossflow level 34 the slope of the energy consumption curve 26 further increases. This is detected by the control device 22, meaning that the total energy consumption becomes worse. Thus, in the next step the crossflow level is again increased to the crossflow level 32. In the result the energy consumption curve 26 again has a smaller slope, i.e., the increase of energy consumption E over time is reduced. With further increasing the crossflow level to the crossflow level 30 the energy consumption curve 26 again becomes flat, i.e., substantially horizontal. Thus, the optimum for the crossflow setting is found, since a minimum energy consumption having relatively steady process conditions is found, and the energy consumption increases very slowly from filtration cycle to filtration cycle. This is the optimum for energy consumption, since it is the minimum which substantially can be kept constant over time. Such crossflow setting, i.e., finding the optimum crossflow level may be carried out by the control device 22 from time to time or during a setup procedure of the filtration device, for example after a cleaning in place. Under relatively steady process conditions the optimum crossflow setting changes slowly. With progressing fouling the crossflow setting may be adapted, i.e., a higher crossflow level may be required. Thus, the setting of the crossflow may be repeated in certain time intervals to again optimize the setting.

[0044] In the embodiment discussed with reference to FIGS. 2 and 3 the crossflow c along the entrance side 4, in particular a crossflow c.sub.1 provided by the recirculation pump 18 has been considered. However, a crossflow may be provided in another way, for example by rotation of the membrane 2 or air scouring. Those methods or systems of continuous physical cleaning may be regarded as an equivalent to a crossflow and the setting of the air flow or rotational speed of the membrane may be carried out in the same way as discussed before.

[0045] Furthermore, according to the invention a recovery level may define the crossflow c. The recovery level or recovery rate is defined as a ratio of the permeate flow in the permeate outlet 8 to the feed flow, i.e., the volume flow in the inlet line 10. As can be seen in FIG. 4 the energy consumption has a minimum for a certain recovery level, in this example for a recovery level of approximately 70%. If the recovery level is too low, energy is wasted since waste amounts of brine or retentate must be treated, resulting in higher costs for brine treatment. If the recovery level is too high the higher recovery pushes the system to its limits which results in high energy and cleaning costs. For example, an excessive energy consumption for the feed pump 12 and/or an excessive fouling may occur, requiring a longer or more intensive cleaning like cleaning in place. The recovery level is also defined as a crossflow in the meaning of the present invention since it depends on the crossflow along the entrance side 4 of the membrane 2. The setting of the recovery level may be done in a similar manner by the control device 22 as the setting of the crossflow as explained with reference to FIG. 3.

[0046] The setting of the recovery level is explained with reference to FIG. 5 in more detail. FIG. 5 shows the total energy consumption E over time t, as shown in the diagram according to FIG. 3. Also, in this diagram the upper curves 26 are energy curves being trajectories discovered during several filtration cycles. In this case three filtration cycles, each consisting of a production period and a following flushing, are regarded. These filtration cycles are shown as dots on the curves 26. The lower curve in FIG. 5 represents different recovery levels 34, 36, 38 and 40. The optimum recovery level is found in an iterative process. The process is started with a relatively low recovery level 34. Several filtration cycles, in this case three filtration cycles, are carried out with this first recovery level 34 and the resulting energy consumption curve or trajectory 26 is discovered. The curve 26 has a small slope. In the next step the recovery level is increased to recovery level 36 and again the energy or cost trajectory 26 is discovered. As can be seen in FIG. 5, the slope of this trajectory 26 is lower, substantially zero. In the next step the recovery level is further increased to a recovery level 38. Then, the resulting energy trajectory 26 again has a higher slope. When further increasing the recovery level to recovery level 40, the slope of the trajectory 26 becomes even greater. This means with this setting the recovery level moves away from the optimum. Thus, in the next step the recovery level is decreased again to the recovery level 38 with the slope of the trajectory 26 decreasing again. The optimum is found for the highest recovery level for which the costs or energy curve or trajectory 26 remains substantially horizontal, i.e., the slope is substantially zero. For this recovery level the costs or energy consumption maintains substantially constant over time. The setup process to find the optimum recovery level may be carried out by the control device 22 in predefined time intervals or for example during a setup process, for example after a cleaning in place.

[0047] As can be seen in the examples according to FIGS. 3 and 5 according to the invention the crossflow is optimized under cost and energy consumption perspectives allowing to increase the efficiency of the entire filtering process.

[0048] While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.

LIST OF REFERENCE CHARACTERS

[0049] 2 membrane [0050] 4 entrance side [0051] 6 outlet side [0052] 8 permeate outlet [0053] 10 feed line [0054] 12 feed pump [0055] 14 concentrate or retentate outlet [0056] 15 concentrate drain [0057] 16 recirculation line [0058] 18 crossflow pump [0059] 20 valve [0060] 22 control device [0061] 24 flow sensor [0062] 26 energy consumption curve/energy consumption trajectory [0063] 28, 30, 32 crossflow level [0064] 34, 36, 38, 40 recovery level [0065] E energy consumption/relative energy consumption [0066] t time [0067] c, c.sub.1, c.sub.2 crossflow [0068] A, B, C, D crossflow level