Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
  • B01D65/10—Testing of membranes or membrane apparatus; Detecting or repairing leaks
  • B01D65/109—Testing of membrane fouling or clogging, e.g. amount or affinity
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
  • B01D65/10—Testing of membranes or membrane apparatus; Detecting or repairing leaks
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D61/00—Processes of separation using semi-permeable membranes, e.g. dialysis, osmosis or ultrafiltration; Apparatus, accessories or auxiliary operations specially adapted therefor
  • B01D61/02—Reverse osmosis; Hyperfiltration ; Nanofiltration
  • B01D61/12—Controlling or regulating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D65/00—Accessories or auxiliary operations, in general, for separation processes or apparatus using semi-permeable membranes
  • B01D65/08—Prevention of membrane fouling or of concentration polarisation
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2321/00—Details relating to membrane cleaning, regeneration, sterilization or to the prevention of fouling
  • B01D2321/40—Automatic control of cleaning processes
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/44—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis
  • C02F1/441—Treatment of water, waste water, or sewage by dialysis, osmosis or reverse osmosis by reverse osmosis

Definitions

• the present invention relates to a method for early detection of the occurrence of scaling in the purification of water by means of a purification plant having one or more membrane elements.
• dissolved salts are withheld to a large extent by the membranes, concentrated and discharged in the membrane concentrate.
• the degree of thickening depends on the conversion of the membrane system; the conversion is the percentage of feed that is converted into product (permeate). Because of said thickening anorganic compounds that are soluble to a limited extent may exceed their solubility product in the concentrate and precipitate on the membrane surface. As a result a layer of solid crystalline material is formed on the membrane surface (scaling). Common compounds that may precipitate include calcium carbonate, barium sulphate, silicate compounds and calcium phosphate. Scaling is highly undesirable because it results in an increase of the resistance of the membrane, so that the pressure has to be increased in order to maintain the production capacity. As a result the energy consumption increases. Moreover the membranes have to be cleaned frequently and the lifespan of the membranes may be shortened.
• Van de Lisdonk et al [2000] describe a method by which the occurrence of scaling can be detected early. According to said method in a water purification plant having membrane elements a small test unit is used wherein a part of the membrane concentrate from the plant is passed through a membrane element. This membrane realises an extra conversion.
• the invention provides a method for early detection of the occurrence of scaling in the purification of water by means of a purification plant having one or more membrane elements, wherein the water to be purified is supplied to the first membrane element of the membrane plant, in each membrane element the water supplied (the feed) is guided past a membrane, that allows a part of the supplied water to pass but withholds the salts dissolved in the water to a large extent, so that the water supplied is separated in the membrane element in a permeate, consisting of the water that has passed the membrane, and a concentrate, consisting of the water that has not passed the membrane including the withheld salts, such that the concentration of the salts in the concentrate is increased with respect to the water supplied to the membrane element, the concentrate of each membrane element is supplied as feed to the next element, a part of the concentrate of the last membrane element is supplied to a scaling monitor containing a same type of membrane element as the membrane elements of the membrane plant, the flux (the volume of permeate per unit of membrane surface) and the conversion (ratio between perme
• the flow rates of the feed and permeate and the temperature and electric conductivity of the feed are measured con- tinuously, and the mass transfer coefficient (MTC) is calculated from said data, characterized in that, the flux and the conversion in the scaling monitor are set such that for a certain selected ion the concentration at the membrane surface at the concentrate side in the membrane element of the scaling monitor (c m SG ) is equal to the concentration of said ion at the membrane surface at the concentrate side in the last membrane element of the membrane plant (c m i ) multiplied by a safety factor (k), equal or larger than 1 , said concentrations being calculable with the boundary layer model described herein.
• MTC mass transfer coefficient
• concentration polarisation concentration polarisation
• the present invention further relates to a method for purifying water by means of a membrane plant having one or more membrane elements, wherein the above-mentioned method for early detection of the occurrence of scaling is used.
• the membrane plant is a plant for nano-filtration or reverse osmosis.
• a number of spiral wound membranes are accommodated in one pressure vessel.
• the plant comprises several pressure vessels connected in parallel having spiral wound membranes.
• the concentrate flows of a number of pressure vessels from the same step are then combined and supplied as feed to one next step in which fewer pressure vessels are connected in parallel than in the previous step.
• the last step of a membrane plant comprises several pressure vessels the operation conditions in the pressure vessels will in general be the same so that a scaling monitor has to be connected to the concentrate of only one pressure vessel.
• the scaling monitor has to be connected to the pressure vessel in which the largest danger of scaling can be expected.
• the desired concentration of the selected ion at the membrane wall at the concentrate side of the monitor can be obtained by the setting of the flux and/or the longitudinal flow rate of the concentrate in the monitor.
• the same type of membrane element here means a membrane element in which the conditions for scaling are the same, that means that the same degree of concentration polarisation will occur.
• a spiral wound element will in general also be used in the scaling monitor. The dimensions need not be the same. Often the element of the scaling monitor will be smaller than the elements of the membrane plant. It is not necessary either to use a same type of membrane material.
• F Faraday constant (96500) [C/mole]
• F w Water flux [m 3 /m 2 s]
• MTC Mass transfer coefficient through the membrane S[m/(s.bar)]
• n Number of membrane envelopes in a module [-]
• TDS Total Dissolved Solids
• total salt concentration [n nole/l]
• u Longitudinal flow rate of the concentrate in the feed con- centrate duct [m/$]
• Temperature of the concentrate (T c ), to be determined by means of temperature meter;
• Concentrate pressure and permeate pressure (P c and P p ), to be determined by means of a manometer; 4. Concentrate flow rate (Q c ) to be determined by means of a flow meter;
• Mass transfer coefficient (MTC) of the last membrane from the membrane plant The MTC of the membrane can be calculated from the test data of the supplier or can be measured. How the MTC can be measured and calculated is described by Van de Lisdonk et al [2000].
• U-value of the membrane can be obtained from the membrane supplier or can be measured, [Verdouw, 1 997]
• Reference temperature usually 10°C (T ref ).
• the degree of concentration polarisation is specific to the ion and is determined by the design of the plant, i.e, by the flux and the flow rate of the concentrate past the membrane.
• the degree of concentration polarisation can be calculated by means of a mass balance of the ion over the boundary layer at the membrane wall and is described by Schock and Miquel [1 987; also see Marinas, 1 996].
• the concentration polarisation factor /?i of ion i is calculation by means of formula (1 ): (1 )
• F w is the water flux through the membrane (Van de Lisdonk et al, 2000) and is calculated by means of formula (2):
• the osmotic pressure at the membrane wall (mrt) can be calculated from the salt composition of the water at the membrane wall, for instance according to Du Pont (1980). Said composition can be calculated by calculating the concentration at the membrane wall (c m ) for each ion from the concentration in the concentrate flow (c c ) and the individual con- centration polarisation factor of said ion ⁇ ) according to formula (3):
• the composition has to be determined as elaborate as possible.
• concentration c m will in any case have to be calculated for the most common ions, in case of drinking water that will at least be Ca 2+ , Na + , Mg 2+ , CI “ , SO 4 2" and HCO 3 " , and ions that may lead to the formation of potential scaling salts, including Ba 2+ and
• the diffusion coefficient Di of ion i is a function of the temperature and the total salt concentration (TDS) in the concentrate and can be determined by means of the Nernst law [CRC Handbook of Chemistry and Physics, 76th edition, 1 995]. First the molar conductivity ⁇ i of ion i is calculated by means of formula (5) and then D, is calculated by means of formula (6) :
• Z is approximately 1 x 1 0 "4 for monovalent ions and 4. 10 "4 for bivalent or trivalent ions [Chang 1977].
• the Sherwood relation is described by Schock and Miquel [1 987].
• Aeyelts Averink [1 993] describes the various experimentally determined relations for spiral wound elements.
• the longitudinal flow rate of the membrane concentrate u can be cal- culated by means of formula (8) [Schock and Miquel, 1 987]:
• the concentrate composition at the membrane side calculated above after being multiplied by the safety factor is now also set at the membrane wall at the concentrate side of the membrane in the monitor.
• the following input parameters are necessary:
• Bulk feed water composition salts scaling monitor is bulk concentrate composition from the plant to be monitored (c c i ) to be determined by means of analyses;'
• Temperature of the bulk feed water composition is temperature of the concentrate of the plant to be monitored (T c ), to be determined by means of temperature meter;
• a SG Surface (A SG ), Length (L SG ) and width (B SG ) of an envelope, number of envelopes (n SG ), diameter of a spacer wire (d f SG ) and porosity of the spacer (e SG ). Said data can be obtained from the supplier.
• Mass transfer coefficient (MTC) of the element from the scaling monitor The MTC of the membrane can be calculated from test data from the supplier or can be measured. How to measure and calculate the MTC is described by Van de Lisdonk et al [2000]. 5. U-value of the membrane from the scaling monitor, can be obtained from the membrane supplier or can be measured [Verdouw 1 997] 6. Reference temperature, usually 10°C (T ref ); 7. Safety factor (k) to be set;
• control parameter for instance calcium, barium, iron or another
• Average retention (Ret) of the ions the percentage of feed con- centration that is withheld by the membrane, can be obtained from the supplier.
• any ion could be chosen.
• concentration in the bulk of the concentrate will be a little lower. Because some salt will always pass the membrane, it is possible that the concentration at the membrane wall at the concentrate side of the element in the scaling monitor will be set too low.
• This can be solved by on the one hand selecting only ions having a valency of 2 or higher, such as Ca, Mg, Ba, Sr, Fe, Mn, or SO 4 or PO 4 , which in general have a rather high retention (approximately > 0.8 in case of nano-filtration and > 0.9 in case of reverse osmosis).
• the average retention of the selected ion is entered when calculating the concentration in the bulk of the concentrate of the scaling monitor.
• the k-value may suitably be selected between approximately 1 .05 and 1 .5.
• a safety factor of 1 .05 can suitably be selected.
• the feed water type is not constant (changing temperature and composition) it is recommended to select a safety factor of 1 .3-1 .5.
• the concentration of ion i at the membrane side of the scaling monitor c m SG ⁇ i has to be:
• the water flux in the scaling monitor F w SG can be set at the same level as the water flux (at the concentrate side) in the last element of the membrane plant to be monitored.
• the wanted concentration can then be obtained by varying the feed flow rate of the scaling monitor. It is also possible to set the longitudinal flow rate of the concentrate in the scaling monitor constant and to set the wanted concentration by varying the flux. Below setting the water flux in the scaling monitor at the same level as the water flux at the concentrate side of the membrane plant to be monitored, was opted for.
• the feed flow rate of the scaling monitor has to be iteratively determined.
• the necessary longitudinal flow rate u of the membrane concentrate can now be calculated back by means of formulas ( 1 ), (4), (5), (6) and (7) using the input parameters of the scaling monitor.
• O-v.SG O- .sG + F w A SG ( 1 3)
• the concentration of ion i in the bulk of the concentrate of the scaling monitor c c SG can now be calculated again by means of formula (10) . Subsequently the concentration polarisation factor ⁇ SG , the necessary longitudinal flow rate of the concentrate u, the concentrate flow rate Q c SG and the feed flow rate Q v SG can be calculated again. Said procedure is repeated until the values converge.
• MTC membrane element of the scaling monitor
• the osmotic pressure can be approximated by measuring the electric conductance and convert it into osmotic pressure (by means of the DuPont method (DuPont, 1 980)).
• the MTC is then calculated by means of the formula:
• measures can be taken such as: lowering the conversion, lowering the dose of acid and/or anti-sealant, or cleaning the membranes.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Water Supply & Treatment (AREA)
• Nanotechnology (AREA)
• Separation Using Semi-Permeable Membranes (AREA)

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Cited By (9)

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Citations (2)

• 2000
  • 2000-10-02 NL NL1016306A patent/NL1016306C2/nl not_active IP Right Cessation
• 2001
  • 2001-10-02 AU AU2002211081A patent/AU2002211081A1/en not_active Abandoned
  • 2001-10-02 WO PCT/NL2001/000724 patent/WO2002028517A1/en active Application Filing

Patent Citations (2)

Non-Patent Citations (4)

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