WO2002028517A1

WO2002028517A1 - Method for early detection of the occurrence of scaling in the purification of water

Info

Publication number: WO2002028517A1
Application number: PCT/NL2001/000724
Authority: WO
Inventors: Carolus Antonius Cornelis Van De Lisdonk; Johannes Cornelis Schippers
Original assignee: Kiwa N.V.
Priority date: 2000-10-02
Filing date: 2001-10-02
Publication date: 2002-04-11
Also published as: NL1016306C2; NL1016306A1; AU2002211081A1

Abstract

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, wherein 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, wherein the flux and the conversion in the scaling monitor are set such that the concentration at the membrane surface at the concentrate side in the membrane element of the scaling monitor for a certain selected ion 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 multiplied by a safety factor, equal or larger than 1.

Description

Method for early detection of the occurrence of scaling in the purification of water

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.

In nano-filtration systems and reverse osmosis systems 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.

Because of said extra conversion it is expected that scaling first occurs at this membrane element, because in this element the salts are after all further concentrated and the over-saturation of poorly soluble compounds increases. The flux (production per m² of membrane surface) of the element, normalised for pressure and temperature, i.e. the mass transfer coefficient (MTC), is continuously measured. By means of the MTC it can be assessed whether scaling occurs and whether measures against this membrane pollution have to be taken. The nature of the scaling can be determined by removing the membrane from the test unit, opening it and analysing it.

It has appeared that by means of the known method described above it is possible to detect the occurrence of scaling in the last step of a plant for nano-filtration or reverse osmosis, early and continuously. It has now been found that by using said method under certain conditions the occurrence of scaling is detected, whereas in fact there is no danger of scaling in the plant to be monitored. When for instance the conversion of the plant to be monitored is set at 85%, the thickening of poorly soluble salts in the concentrate is approximately a factor 6.7. In the test unit connected to the plant the total conversion (plant + test unit) increases up to approximately 88-99%. This means that the thickening of poorly soluble salts in the test unit increases from 6.7 to 8.3-10. Said increase is so large that the results obtained by means of the test unit are no longer representative for the membrane plant to be monitored. The danger of scaling is then overrated as a result of which measures against scaling might be taken at a moment it is not yet necessary. This may lead to unnecessary costs and loss of production.

Therefore there is a need for a method for early detection of scaling by means of scaling monitor as described above, with which also in case of a higher conversion of the system a reliable indication of the danger of the occurrence of scaling can be obtained. It has now been found that said need can be provided for by basing the setting of the above-mentioned scaling monitor on the concentration of a certain selected ion at the membrane surface at the concentrate side in the last element of the plant. Said concentrate is calculated by using a boun- dary layer model by means of which the concentration polarisation at the membrane can be calculated.

Thus 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 permeate and feed) in the scaling monitor are set such that the possibility of scaling in this element corresponds to or is larger than that in the last membrane element, the pressure of the feed, pressure drop over feed concentrate duct

(or concentrate pressure), 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.

The withholding of the dissolved salts in the membrane concentrate by the membrane, results in a higher concentration of dissolved salts at the membrane wall than in the bulk of the membrane concentrate. This is referred to as concentration polarisation. The increase in concentration may amount to approximately 1.1 -1.6 times the concentration in the bulk of the concentrate. The concentration of poorly soluble salts and thus the possibility of scaling therefore is larger on the membrane wall than in the bulk of the concentrate.

By basing the setting of the monitor according to the invention on the calculated concentrations on the membrane wall at the concentrate side of the plant to be monitored, the possibility of scaling in the monitor is the same or just a little higher than (depending on the safety factor) in the plant to be monitored. By using this setting it is achieved that the extra conversion in the monitor is limited as a result of which the results are more reliable.

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. According to a preferred embodiment of the invention, the membrane plant is a plant for nano-filtration or reverse osmosis.

In a commonly used type of membrane purification plant, a number of spiral wound membranes are accommodated in one pressure vessel. Often 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. When 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. When in such a case the conditions are not the same in all vessels of the last step, 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. When for instance the membrane plant contains spiral wound elements, 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.

Elucidation on calculating settings Below will be shown how the parameters of the setting of the scaling monitor can be obtained. The calculation of said parameters takes place in two steps. First the concentration at the membrane wall c_{m i} of the ions in the concentrate flow at the concentrate side of the last membrane element in the membrane plant, is calculated. By means of the selected safety factor the required concentration at the membrane wall at the concentration side of the membrane element in the scaling monitor (c_{m SG}) is obtained therefrom. In the second step the setting parameters of the scaling monitor are calculated from said latter value. Finally it will be indicated how the mass transport coefficient MTC of the scaling monitor can be determined.

The calculations below relate to a plant having spiral wound membrane elements, in which the scaling monitor as well contains a spiral wound membrane.

The symbols used in the following calculations have the following meaning:

A = Membrane surface [m²]

B = Width of a membrane envelope [m] c, = Concentration of ion i [mole/l] d_h = Hydraulic diameter [m] d_f = Diameter of the filaments of the spacer [m]

D| = Diffusion coefficient of ion i [m²/s]

F = Faraday constant (96500) [C/mole] F_w = Water flux [m³/m²s] k = Safety factor [-] k, = Mass transfer coefficient of ion i in the boundary layer [m/s]

L = Length membrane envelope [m]

MTC = Mass transfer coefficient through the membrane S[m/(s.bar)] n = Number of membrane envelopes in a module [-]

P = Pressure [bar]

Q = Flow rate [m³/s] ^• R Gas constant 8,31 [J/mole. K]

Ret, Retention of ton i [-] Sh, Sherwood number of ion i [-] T Temperature f °C]

TDS "Total Dissolved Solids", total salt concentration [n nole/l] u Longitudinal flow rate of the concentrate in the feed con- centrate duct [m/$]

U Membrane dependent temperature constant [-] z valency ion Z Constant in the mplar conductivity equation [-]

Greek

_n concentration polarisation factor of ion i [-]

(volume feed concentrate dugt volume spacer)/vdlurηe feed concentrate duct [-3

Molar conductivity [S,mo|e².mole^"'¹]

M Molar conductivity in case of endless thinning lS.mole².mole"¹} 1 Viscosity [Pa.s]

Osmotic pressure [bar]

Subscripts c = concentrate

V =?= feed m = at membrane surface

P permeate rσf = In reference conditions sg scaling monitor

Calcuiation of the water composition at the membrane wall at the con- i centrate side of the plant to be monitored.

For the calculation of this water composition the "Scaling 'Prediction Model" is used.

Necessary input of the model:

1 . Bulk concentrate composition salts (C_{c l}) to be determined by means of analysis;

2. Temperature of the concentrate (T_c), to be determined by means of temperature meter;

3. 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;

5. Dimensions of the used membranes; Length (L) and width (B) of an envelope, number of envelopes (n), diameter of a spacer wire (d_f) and porosity of the spacer (_e). Said data can be obtained from the supplier.

6. 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].

7. U-value of the membrane, can be obtained from the membrane supplier or can be measured, [Verdouw, 1 997]

8. Reference temperature, usually 10°C (T_ref).

For each ion the degree of concentration polarisation ( ?,) is calculated. 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 )

A = ∞p(-^w-^{' rf}*

D_rSh,

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 {β ) according to formula (3):

Because the value of F_w is necessary in order to be able to determine β\ and F_w indirectly is a function of β again, iteration is necessary.

For the calculation of the osmotic pressure π_m the composition has to be determined as elaborate as possible. The 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 Ca²⁺, Na⁺, Mg²⁺, CI^", SO₄ ^2" and HCO₃ ^", and ions that may lead to the formation of potential scaling salts, including Ba²⁺ and

PO ⁾,²³-

The hydraulic diameter (d_h) can be calculated from the dimensions of the membrane by means of formula (4) [Schock and Miquel, 1 987]: _j 4 - ε (4) ^d" ⁼ 2 _n . 4

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 Λi of ion i is calculated by means of formula (5) and then D, is calculated by means of formula (6) :

λι = Λ° - Z-JTDS (5)

The value of Z is approximately 1 x 1 0^"4 for monovalent ions and 4. 10^"4 for bivalent or trivalent ions [Chang 1977].

The Sherwood number Sh indicates the relation between the Reynolds number Re and the Schmidt number Sc (Sh = a.Re^b.Sc^c). 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 coefficients a, b and c used in the formula (7) below are experimentally determined:

(7) h_i = 0,12 • (^{p ' u ' d}* ) o.⁴ⁱ . (_ZL_) o." 1 P -D,

The longitudinal flow rate of the membrane concentrate u can be cal- culated by means of formula (8) [Schock and Miquel, 1 987]:

(8)

Q„

2 • d _j -n -L - ε

By repeating this procedure for each ion the concentrate composition at the membrane wall is obtained.

Calculation settings monitor

According to the invention 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. For calculating the setting of the scaling monitor the following input parameters are necessary:

1 . 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;'

2. 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;

3. Dimensions of the membrane used in the scaling monitor: 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.

4. 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;

8. Concentration of control parameter, for instance calcium, barium, iron or another;

9. Average retention (Ret) of the ions, the percentage of feed con- centration that is withheld by the membrane, can be obtained from the supplier.

As regards the choice of the ion as control parameter: When the retention of the membrane of the scaling monitor would be 1 00% any ion could be chosen. In practice the 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 SO₄ or PO₄, which in general have a rather high retention (approximately > 0.8 in case of nano-filtration and > 0.9 in case of reverse osmosis). On the other hand 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. In a stable feed water type (almost constant temperature and salt composition) a safety factor of 1 .05 can suitably be selected. When the feed water type is not constant (changing temperature and composition) it is recommended to select a safety factor of 1 .3-1 .5.

Using c_{m i} as control parameter for the selected ion i, the concentration of ion i at the membrane side of the scaling monitor c_{m SGιi} according to formula (9) has to be:

'm.SG = C„ (9) The concentration of ion i in the bulk of the concentrate in the scaling monitor c_{c SGιi} according to formula ( 1 0) is approximately equal to:

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.

First the concentration polarisation factor at the concentrate side of the membrane in the scaling monitor is calculated by means of formula ( 1 1 ):

- ,SG ( 1 1 ) i ' SG ^cc,sσ

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.

The necessary concentrate flow rate of the scaling monitor Q_CιSG according to formula ( 1 2) will then be:

^Q _C,_SG = ^U _SG • ²-^df.sG • n_SG . L_SG . e_SG ( 12) The feed flow rate of the scaling monitor Q_{v SG} according to formula ( 1 3) then is:

O-v.SG = O- .sG ^{+ F}w ^ASG ( 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 β _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.

By now setting the scaling monitor such that the calculated feed flow rate and water flux are obtained, a concentration at the concentrate side of the membrane element will be obtained in the scaling monitor that is equal to the corresponding concentration in the membrane plant to be monitored multiplied by the safety factor.

In order to establish whether danger of scaling formation may arise in the membrane plant, the continuous course of the mass transfer coefficient

(MTC) of the membrane element of the scaling monitor can be followed. When said MTC drops, pollution of the membrane apparently occurs, which quite likely will be caused by the precipitation of poorly soluble salts.

In order to be able to follow the MTC of the membrane element continuously, a continuous accurate determination of the feed pressure P_V;SG' concentrate pressure P_C,SG, product flow rate Q_P,_SG; and temperature and osmotic pressure in feed and concentrate is required.

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:

When the MTC of the membrane element of the scaling monitor drops, measures can be taken such as: lowering the conversion, lowering the dose of acid and/or anti-sealant, or cleaning the membranes.

Due to the scaling monitors said measures can be taken timely. Said measures however lead to a production loss and/or entail additional costs

(chemicals, cleaning, energy). With the present invention it is prevented that such measures are taken too early so that unnecessary costs are avoided.

Literature

Bakish, R. Theory and Practice of reverse osmosis, International

Desalination Association No. 1 1 985

Chang, R: "Physical chemistry with applications to biological systems"

MacMillan Publishing Co. New York, ISBN 0-2-321020-6, 1 977

Du Pont, Technical information manual, USA, 1 980

Lisdonk, C.A.C. van de, Paassen, J.A.M. van, Schippers J.C. : "ScaleGuard signaleert vroegtijdig scaling by nanofiftratie en omgekeerde osmose ", H₂0, No. 6 2000 Lisdonk, C.A.C. van de, Paassen, J.A.M. van, Schippers J.C: "Monitoring scaling in nanofiltration and reverse osmosis membrane systems" submitted for publication in the proceedings of the conference Membranes in Drinking and Industrial Water Production, 2-6 October, Paris.

Marinas, B.J.: "Modelling Concentration-Polarisation in Reverse Osmosis Spiral Wound Elements", Journal of Environmental Engineering, April 1 996

Schock, G. and Miquel, A.: "Mass transfer and pressure loss in spiral wound modules ", desalination 64, 1 987

Verdouw, J. Folmer, H.. proceedings Membrane Technology Conference, New Orleans, 1 997

Wangnick, W. 2000 IDA Worldwide Desalting Plant Inventory Report no.

Claims

1 . 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 permeate and feed) in the scaling monitor are set such that the possibility of scaling in this element corresponds to or is larger than that in the last membrane element, the pressure of the feed, pressure drop over feed concentrate duct (or concentrate pressure), the flow rates of the feed and permeate and the temperature and electric conductivity of the feed, are measured continuously, 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 concentration being calculable with the boundary layer model described herein.

2. A method according to claim 1 , wherein the membrane plant is a plant for nano-filtration or reverse osmosis.

3. A method according to any one of the preceding claims, wherein the membrane plant at least comprises one pressure vessel in which spiral wound membranes are accommodated.

4. A method according to any one of the preceding claims, wherein the safety factor ranges between 1 .05 and 1 .5.

5. A method according to any one of the preceding claims, wherein the selected ion has a valency of 2 or higher.

6. A method for purifying water by means of a membrane plant having one or more membrane elements, wherein the method for early detection of the occurrence of scaling according to one of the claims 1 -5 is used.