Patent Publication Number: US-2011049050-A1

Title: Method of controlling a separation unit having a reverse osmosis element and reverse osmosis unit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of priority of German Application No. 102009040049.4, filed Sep. 3, 2009. The entire text of the priority application is incorporated herein by reference in its entirety. 
     FIELD OF THE DISCLOSURE 
     The disclosure relates to a method of controlling a separation unit having a reverse osmosis element and to such a separation unit. 
     A separation unit and a method of this type are known from WO 96/41675. The prior method is to be able to cope with changing work loads of the separation unit, namely with strongly varying raw water quantities, while simultaneously allowing a substantially constant yield, i.e. a predetermined ratio between permeate and concentrate. To this end, the flow of the concentrate is automatically adapted to the raw water influx, viz. in such a way that these two flows have and maintain a constant ratio relative to one another. Thus it is achieved that the recirculation of concentrate into the raw water influx is automatically adapted even if the utilization of capacity varies. The raw water influx is detected by a flow meter disposed upstream of the pump that provides for the filter pressure, i.e. ahead of this pump, and is used to control the flow cross-section and flow resistance in the concentrate outlet conduit. The capacity of the pump, however, remains unchanged, i.e. the pump is controlled to maintain a constant pressure. Although this step helps to achieve a constant yield apart of the pump&#39;s capacity is given away. This process is particularly fitted for units having smaller throughputs. 
     When the raw water pump is controlled to have a constant admission pressure, for instance, changes of the raw water temperature are not taken into account. If the raw water pump is controlled to still maintain a constant pressure although the raw water temperature has changed, the permeate and concentrate flow is either reduced or increased depending on the temperature change. As a consequence the yield is misaligned which, in an extreme case, may lead to abrupt precipitations of salt in the concentrate. These abrupt precipitations clog the membrane, which necessitates a time-consuming and laborious cleaning. 
     SUMMARY OF THE DISCLOSURE 
     The disclosure has as one aspect to provide a method and a separation unit for reverse osmosis filtration, which allow in a constructively simple way to keep the yield constant despite, for instance, temperature and pressure variations in the inlet, without requiring to this end a buffer container for the forerun. 
     Moreover, by the quality-dependent rinsing of the unit with permeate not only water is saved, but also the quantity of used cleansing chemicals. 
     By controlling the raw water pump in accordance with the disclosure based on the flow generated by the pump it is ensured in a constructively simple and very effective and sensitive way that the permeation through the membrane can be kept constant without being dependent on external influences, no that a yield set in advance, i.e. a ratio of permeate quantity to concentrate quantity set in advance is obtained automatically and is kept constant. This is accomplished by keeping the influx quantity and concentrate quantity constant. 
     The yield can be varied by changing the flow cross-section of the concentrate conduit in response to a flow measurement performed in the conduit. 
     Furthermore, it is possible to vary the permeate quantity within a bandwidth in order to obtain, for instance, a level control for a permeate tank. This has the additional advantage that the reverse osmosis unit thus achieves optimum running times (shut down and start-up are minimized, thereby increasing the life cycle of the membrane). 
     Another advantage afforded by the unit according to the disclosure is a quality-dependent cleaning, with the cleaning success being actually monitored (and not being automatically assumed after a predetermined time lapse), which is preferably accomplished by a conductivity measurement in the concentrate outlet conduit. This measure cannot only be used for the disclosed separation unit and the disclosed method comprising a flow-controlled pump, but also for differently constructed separation units and methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       An embodiment of the disclosure shall be explained in more detail below by means of a single drawing. 
       The FIGURE shows a schematic representation of a separation unit  1  according to the disclosure for performing the method according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The separation unit  1  comprises a conventionally constructed reverse osmosis element  2  having, for instance, a plurality of membranes, whose permeability is adjusted to the molecules to be filtered out. In the embodiment shown a single aggregate can be seen. The unit also works with the usual multiple aggregates, however. 
     The reverse osmosis element  2  is provided with the raw water to be filtered through a conduit  3 . As is common, the raw water conduit  3  first passes a prefilter  4  in which coarser mechanical impurities are removed. At the same time, the hardness is stabilized by means of a correspondingly constructed device  5 . The raw water then flows into a pump complex  6  which comprises an ordinary high-pressure pump with a frequency converter. The pump provides for the predetermined volume flow. After leaving the pump complex, the raw water flows via a temperature measuring means  7 , a conductivity measuring means  8  into a commonly and optionally constructed flow meter  9 . Preferably, the flow meter  9  is an inductive flow meter (IDM). The flow meter  9  is connected via a signal line  10  to the pump complex  6 . Downstream of the flow meter the raw water flows into the reverse osmosis element  2 . 
     A permeate conduit  11  for the cleaned water and a concentrate conduit  12  for salt-loaded liquid of an increased concentration come out of the reverse osmosis element  2 . The concentrate conduit  12  continues via another flow meter  13 , which again is preferably an inductive flow meter (IDM), a control valve  14  and a conductivity measuring means  15  until it reaches an outlet channel  16 . 
     The flow meter  13  in the concentrate conduit  12  communicates with the control valve  14  by a signal line  17 , so that the control valve  14  may be controlled in response to the flow measurement. 
     The permeate conduit  11  continues via a stop valve  18  until it reaches the consumer directly and/or a storage tank  19  in which permeate can be made available. The storage tank  19  is provided with a level gauge  20 ; represented are a lower and an upper level electrode. The level gauge is connected to pump  6  via a signal line  21 . 
     During the operation of the separation unit  1  the desired yield (depending on the type and quantity of the substances contained in the raw water which are to be removed) and the flow required to generate an optimum permeation for obtaining the desired optimum yield are defined first. The operation of the unit is started, and the flow is monitored by the flow meter  9 . If the flow rate changes, e.g. as a result of temperature changes in the raw water influx, the pump is controlled in such a way that the flow reassumes the predetermined value. In the present example this means that due to the temperature change in the raw water influx the viscosity thereof is varied. For instance, a lower temperature implies a higher viscosity of the raw water, which again implies a reduced volume flow being directly associated with the permeation through the membrane. A pump speed now controlled to be higher effects an increased volume flow and thus again a quantitatively balanced permeation through the membrane. 
     The separation unit according to the disclosure and the method according to the disclosure still function in a satisfactory manner even if the permeate quantity is varied for whatever reasons. The bandwidth for such a variation amounts to ±10 to 20%, which can thus for instance be used to maintain the tank  19  always at a certain filling level. The variations in the permeate quantity can be induced, for instance, by varying the flow cross-section and thus the outflow of concentrate through the concentrate conduit  12  by opening or closing the control valve  14  in an adjusted manner. This advantage over conventional systems particularly allows for smaller tank sizes because, as a result of the adaptation of the permeate quantity, faster reactions on varying conditions are feasible even downstream of the separation unit. 
     In a preferred embodiment the flow of the raw water is set at 29.4 m3/h at 10.5 bar at 12° C. With a yield of 85% this value results in an 85% share of permeate and a 15% share of concentrate, i.e. 25 m3/h of permeate at 0.3 bar positive pressure and 4.4 m3/h of concentrate at 7.7 bar positive pressure. If the temperature of the raw water changes, the flow meter maintains the flow rate of the raw water at the value at 12° C., that is, the flow is either reduced or increased, namely in such a way that the yield of 85% of permeate and 15% of concentrate remains constant (while, of course, the quantity of concentrate and permeate varies). This variation can be accomplished within a bandwidth of ±10 to 20%, so that the disclosed separation unit  1  can produce, for instance, between 20 and 31 m3/h of permeate without the yield of 85% undergoing a change. By this it is prevented in a simple way that the concentrate is overloaded and uncontrolled precipitations occur. 
     The separation unit according to the disclosure also allows for an improved cleaning. This cleaning can, however, also be used in other separation units not having a flow-controlled pump. According to the disclosure the cleaning is accomplished in a quality-dependent manner in response to the values measured by the conductivity measuring means  15  in the concentrate conduit  12 . In this way it is reliably determined whether the liquid (concentrate/cleaning liquid/rinsing liquid) flowing off into channel  16  contains a sufficiently low salt content in order to be regarded a clean. By this it is ensured that all salts have been removed and that no residual salts are still in the system as a result of pressure fluctuations. This rinsing alternative saves rinsing liquid and prevents scaling and biofouling in out-of-service times. 
     The FIGURE shows other modifications which may be provided optionally. For instance, the FIGURE includes the possibility to switch the unit over to a conventional control. To this end, an admission pressure sensor  25  is provided upstream of pump  6 , and a pressure sensor  26  is provided downstream of pump  6 . The signals are transmitted via a signal line  27  to allow, if necessary, switching over to admission pressure detection by sensor  25  and to controlling the pressure of pump  6  by pressure sensor  26 . 
     A second optional embodiment relates to the possibility to also rinse the unit with permeate, if necessary. To this end, the unit  1  is supplied by the pump  29  (frequency-controlled) via conduit  28  with permeate from the storage tank  19  (or another source). Conduit  28  leads into the raw water flow preferably upstream of temperature sensor  7 . In this case, too, the rinsing result can be monitored by the conductivity measuring means in the concentrate conduit. 
     The quality control via the concentrate outlet conduit moreover permits a need-based predetermination of rinsing times during the operation. Thus, the optimum service life of the membrane is already determined during the operation so as to reduce already at this point scaling and biofouling caused by too long an operation. Furthermore, avoiding unnecessarily short cycles significantly increases the lifetime of the membrane by the all in all smaller cleaning load. 
     Moreover, another conductivity sensor  30  may be integrated in the permeate conduit  11 , which affords an additional qualitative control of the quality-dependent rinsing with permeate. To this end, the measured conductivity values at conductivity sensors  8  and  30  can be compared. In case of rinsing with permeate, the rinsing thereafter via conduit path  28  and the frequency-controlled pump  29  takes place until the conductivity measurements of the sensors  8  and  30  are identical. 
     Hence, the separation unit provides for a quantity-dependent control of two volume flows, by means of which temperature and admission pressure fluctuations can be compensated. The intervention takes place on the raw water and concentration side, i.e. at reference numbers  9  (flow meter) and  13  (flow meter). Depending on the quality the rinsing may be accomplished with permeate and chemical substances. As compared to conventional, time-controlled rinsing (which is either too short or too long) this rinsing saves water and cleaning agents. Scaling and biofouling are reduced because the rinsing takes not too long. The lifetime of the membrane is prolonged as the cleaning takes place not too often and only when necessary, and only for the necessary length of time. If needed, a switching to conventional control may be performed. The bandwidth of the disclosed unit permits the compensation of raw water fluctuations. Moreover, the filling level in the buffer tank can thus be controlled. Therefore, the same may be smaller than usual. It is an advantage if no excessively great quantities have to be stored in the buffer tank (permeate tank). 
     The disclosure is not limited to the described and illustrated embodiment. The flow-controlled pump can, for instance, also be used in other separation units with other membranes and/or other combinations of membranes. The storage tank including its level control need not necessarily be connected to the pump, but may be controlled separately on the basis of request signals coming from outside the separation unit. Also, the separation unit may comprise any standard and useful components not shown in the FIGURE.