1*

Water engineering department1 UNESCO-IHE, Westvest 7, 2611 AX DelftThe Netherlands

Abstract

this is study for considering how to detect theproblems from analysis the operation data in RO plant, in this study we have applied ASTM method 4516 , wehave introduced a new parameter for measuring theperformance of RO processes, the coefficient ofperformance, which is considered as a tool foridentifying the plant status. The thermodynamictreatment for reverse osmosis processes depends on thebalance of chemical potential of the solvents on eitherside of the membrane. As a general rule, the drivingforce for the reverse osmosis process and the osmosisprocess is the difference between the chemical potentialof the solvents on either side of the membrane. Inreverse osmosis processes, the chemical potential of thesolvent in the saline water is increased by increasingthe pressure; as we know, the more dissolved solids,there are in water, the less is the chemical potential ofthe solvent. The effect desired, in reverse osmosisprocesses, depends on the difference in the solventchemical potential function. The coefficient ofperformance is the effect desired divided by the energyrequired (The coefficient of performance (COP) isdefined as the ratio of the theoretical work to the actualwork required), and this coefficient of performancegives us a complete description of the reverse osmosisprocess efficiency, and is a helpful tool for diagnosis ofthe problems in the process. Keywords: coefficient ofperformance, data normalization techniques, efficiency,RO process, solvent chemical potential

1. Introduction:Membrane processes are becoming increasinglyattractive as an alternative to conventional water andwastewater treatment. Membrane filtration techniquesare very promising for the preparation ofmicrobiologically safe and biologically stable drinking

water because of their capacities for removing micro-organisms, and also some inorganic and organiccompounds. However, scaling and membrane foulingare causing serious operational problems. Biofoulingthat is, the growth of biomass and the formation ofbiofilm on the membrane surface, cause flux reductionand/or increased pressure drop in the nanofiltration(NF) and reverse osmosis (RO) processes. To identifythe operational problems the operational data is used.The main parameters for this method are the permeateflow rate (Q), and salt passage (SP). According to theASTM-D4516 method, examination of these twoparameters enables us to discover exactly what hashappened to the membrane. The permeate flow ratereflects the quantity of water produced and the saltpassage reflects the quality of water produced. Anothermethod is the mass transfer coefficient (MTC) method.This method evaluates the change in the mass transfercoefficients for both water and salt over the time ofoperation to find out what has happened to themembrane during the operational period. In yet anothermethod, called the coefficient of performance (COP)method, COP is monitored over the operation period.By observing the decline in COP and also the change inthe normalized permeate flow rate and salt passage, weare able to identify the exact problems. This is a genericmethod.2. Materials and Method:

Calculation method:The coefficient of performance (COP) is defined as theratio of the theoretical work to the actual work requiredto produce a mole of permeate water from the feedwater. The work is evaluated through the difference inchemical potential of water in the feed and thepermeate, since the chemical potential of water is givenby. [7,8]

p

p)dp(T,v+))Xp,(T,(aRT+)p(T,μ=)Xp,(T,μp

wwwwww

0

0 ln(1)

And at the isothermal condition, the chemical potentialdiffe

)P(Pv+)permeateX(XRT permeatefeedw,wfeedw, /ln

Development a Coefficient of Performance for Predicat the ReverseOsmosis Desalination Process Performance

Malik M. A. Fakron

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rence (J/mole) between the feed and permeate is givenby

(2) A Assuming that the activity coefficient is unity andthe molar volume of water is independent of thepressure. The work (J/s) required for the molar

permeation rate of water, , is

(3)

Assumed neglect the effect of the change into thesystem entropy due to the change in concentration, thenenergy input into the system (J/s) is

econcentratteconcentrarfeedfeed PQPQ (4)

Therefore, the COP defines as the ration of the effectdesired which work to the energy input into the systemare.

]PQP[Q)]P(Pv+)X(XRT[=COP cocentratetecocncentrafeedfeedpermeatefeedwm

permeatew,feedw,m //ln (5)

Based on equation (5), COP can be calculatedaccording to the ASTM D 4516 procedure by thefollowing steps [2].

Step 1Calculate the molar permeation rate of water from thevolumetric permeation rate and density of water.

Molar permeation rate of water (mol/s) =standardized volumetric permeation rate(m3/s) *106mole3/m3) * 1.0 (g/cm3) /18.02(g/mol) (6)

Step 2Calculate the Total Dissolved Solute (TDS) from theelectrical conductivity (EC) for both the feed streamand the permeate stream by using the followingequation [12].

EC=TDS *16.4 (7)

Step 3Assuming TDS consists only of NaCl, calculate themolar flow rate of NaCl in both feed and permeatestream using

Molar flow rate of NaCl (mole/s) = Volumetric flowrate (m3/s) * TDS (mg/L = g/m3)/58.5 (g/mole)(8)

Where 58.5 is the formula (molecular) weight of NaCl.Since NaCl is fully dissociated into sodium andchloride ions, the molar flow rate of NaCl is consideredto be equal to the molar flow rate of Na+ ion (n•

Na) andalso Cl- ion (n•

Cl).

Furthermore, the molar flow rate of water, n•w, is

approximated by

Molar flow rate of water (mole/s) = Volumetric flowrate (m3/s)/Molar volume of water, vow (m3/mole).(9)

This approximation can be justified since the molefraction of both Na+ and Cl- ions are much smaller thanthat of water in both feed and permeate.Step 4Calculate the mole fraction of water for both feedstreams and permeate stream using the followingequation.

wClNa

w

wn+n+n

n=X

(10)

Step 5Next, calculate the coefficient of performance COPusing equation (5), where the temperature, T, thevolumetric flow rate of the feed stream and thepermeate stream, Qfeed and Qconcentrate, respectively, andthe pressure of the feed stream and the Permeatestream, Pfeed and Pconcentrate, can be experimentallyobtained.Step 6Plot COP versus time on a graph.Step 7The ASTM D4516 procedure also describes the methodto standardize the permeate flow and salt passage usingthe following equations [2]:

a

apafbapafba

fa

spsfbspsfbs

fs

s Q

)](TCFπ+πPΔP

[P

)](TCFπ+πPΔP

[P=Q

2

2(11)

afs

fa

fba

fbssasas SP)

C

C()

C

C(]TCF[TCF]Q[Q=SP // (12)

In equations (11) and (12), the last subscripts is and aindicate the actual and standard (initial, t =0)conditions. In equation (11), Pf−

ΔPfb2

indicates the

average of the feed and brine (retentate) pressure.Hence, it is quite obvious that the term inside thesquare brackets corresponds to the driving force for thepermeation of water. TCF accounts for the effect oftemperature on the permeation rate of water and can becalculated using )(T=TCF 251.03 , where T is

)P(Pv+)permeateX(XRT permeatefeedwm

,wfeedw,m /ln

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ISSN: 2278-0181

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temperature (o C). Using equation (11), the flow rateobtained under the actual conditions is normalized tocorrespond to the initial conditions. Similarly, the %separation obtained under the actual conditions isnormalized to correspond to the initial conditions usingequation (12).[2]

To identify the problem with the process, we first lookat the COP versus time plot to examine whether COP isdeclining with time. We then identify the period wherethe COP decline occurs. Next, we will plot tostandardized permeate flow decline against time andsalt passage against time to find the specific reason forthe COP decline. COP seems a good indicator for themembrane state and a helpful tool for identifying themembrane or process problems.3. . Case study:To test this method, the data from the brackish waterdesalination plant located in Klazienaveen,Netherlands, was used. The nominal capacity of thebrackish water reverse osmosis (BWRO) plant is 75m3/h (500.000 m3 per year). This test considers only thefirst stage in the first array, which means that the ROmodule capacity is 15 m3/h. The client has a specialwater quality standard. The requirements for theproduct water quality are shown in Table 1.[11]

Table (1) Product water quality requirements [11]

Parameter Unit Process water

Acidity PH 5-7.5

Conductivity µS/cm 20Chloride Mg/l 5

Raw water (surface water) is obtained from canals nearthe site. A major issue is the variation in its qualitybetween summer and winter. In summer, raw water isobtained from a nearby lake. This water has a relativelylow concentration of iron and organic matter, a lowturbidity, and a high concentration of salts. In winter,the quality of the water is quite different, with high ironand organic matter concentrations and a high turbidity.At this time of the year, the water comes from the peatareas in the northeast of the Netherlands and istherefore affected by the peat and humus. Usually, thechange between the two water types is gradual andoccurs in April and October. Table 2 shows thecomposition of the canal water. [11]The process scheme of the industrial plant contains aseries of pretreatment steps prior to the RO system in

which coagulation is promoted using polyaluminumchloride (PACl).The PACl dosage was added to adjust the UV/DOCratio to 2.5 using a UV-monitor on the UF system. PHdecrease by adding PACl. The RO system received theUF permeate as feed; however, it was necessary tocontrol the pH of the RO system through PACl dosageto reduce CaCO3 scaling and Al3+ precipitation on theRO membrane. Additionally, a low dosage ofpolyaluminium chloride (PACl) (3-5 mgAl3+/L) wasneeded to prevent serous membrane fouling and allowfor a stable ultrafiltration operation with a gross flux of80 L/m2.h. Figure 1 shows a schematic view of theplant. The data acquired from this plant during 2005 isused to examine the method described above.[11]

Table (2) Canal water composition [11]

fig.1

The RO membrane characteristics in the Klazienaveenplant are shown in Table 3. A TFC polyamidemembrane in spiral wound configuration was used.

Parameter Unit Summer WinterPH - 6.3 7.5Turbidity NTU 10 50Suspendedsolids

Mg/l 10 30

Iron Mg/l 2 10Manganese Mg/l 0.1 0.3Conductivity µS/cm 650 250Chloride Mg/l 120 60DOC Mg C/l 15 35UV(254nm) ABS/m 50 200

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Table (3) Membrane characteristics [11]

Criteria DescriptionGeneral information

Supplier: HydranauticsType: ESPA2-LD

Diameter: 8 [inch]Area/element:

37,1 [m2 ]

The RO membrane modules are operated in a cross-flow mode. The system consists of three arrays. Asschematically shown in Fig. 2, each array is comprisedof two stages; the first stage with three pressure vesselsand the second stage with two pressure vessels. Eachpressure vessel is 6 meters long and contains 6elements of 1 meter each. The diameter of themembranes is 8 inches. The product water recoveryfrom the reverse osmosis unit is 75%.

Fig.2 Reverse Osmosis membrane module system

It should be noted that the data from the first stage ofthe first array is going to be used to test the validity ofthe method.

5. Results & Discussion:

Figure 3 shows that there was a decline in COP duringthe period from the beginning of January 2005 to theend of July 2005. The COP is gradually decreasingduring this period due to the increase in the feedpressure to maintain the permeate flux at the fixedlevel. Figure 4 shows instability in the normalized

permeate flow rate with time. It also indicates that thepermeate flow rate was above the required 15m3/hduring the entire period.

Fig. 3 COP change during the one year operation period

Fig.4 Normalized permeate flow rate during the one year operationperiod

Fig.5 Normalized salt passage during the one year operation period

Figure 5 illustrates the change in the normalizedsalt passage with time. The normalized salt passagegradually increases during the operational periodand eventually surpasses the imposed salt passagelimit of 3.5. %. The quality of the permeates istherefore, not as good as desired. This may be due

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to damage at the dense selective layer of the ROmembrane caused by befouling or by the cleaningreagent used. This example shows the usefulnessof the method to identify the problem source in theplant operation.First, the COP versus time plot(Fig.3) provides us with general information onwhether the plant is working well.Next, thenormalized permeate flow rate versus time plot(Fig. 4) shows quantitatively if the permeate flowrate stable and sufficient. If not, it has possiblybeen caused by membrane clogging or fouling.Furthermore, the normalized salt passage versustime plot (Fig. 5) shows it quality of the permeatessufficiently well. If not, it is possibly due to thedamage of the dense layer of the ROmembranecaused by biofouling. The biofouling may alsohave caused the reduction in the permeate flowrate. In order to make a better diagnosis, anassembly organic carbon ( (AOC) measurement isnecessary Autopsies of the first and last element ofthe first stage might also be necessary to measureadenosine triphosphate(ATP), total dissolvedcarbon(TDC), and heterotrophic plate count (HPC)to confirm that baffling has indeed occurred.

6. Conclusions:

1. In this study there is new development whichis The Coefficient of performance COP, whichis the useful indicator of whether thedesalination system is operating efficiently. Itapplied as a tool for real time analysis of theplant data to predict for the plant problems andthis the new way to understand how the plantwork. this indicator gives the real reading forthe membrane performance in the plant and itis helpful tool for us to predicate formembrane in early stage ,because thismeasurement gives the real

2. There is a decline in plant performance overtime, and this decline is due to an increase inpermeate water TDS.

3. It is useful to standardize the permeate flux toknow quantitatively if the to permeate flux is

within a desired range during the plantoperation. The decrease in the standardizedpermeates indicates clogging or fouling of themembrane.

4. It is also useful to standardize the salt passageto know if the dense selective layer of themembrane was damaged by biofouling.

5. The poor membrane performance is indeeddue to damage of the dense membrane layerdue to biofouling, and autopsies of themembrane modules are necessary.

6. LIST OF SYMBOLS:7.Aw : Activity of water

Cfa : Feed concentration at actual conditions, mg NaCl/l

Cfs : Feed concentration at standard conditions, mg NaCl/l

Cfba : Feed-brine concentration at actual conditions, mg NaCl/l

Cfbs : Feed-brine concentration at standard conditions, mgNaCl/l

EC : Electrical conductivity（mS/m）

n•w : Molar flow rate of permeate water (mol/s)

nH2O :Molar flow rate of water (mol/s)

nNa : Molar flow rate of Na+ (mol/s)

nCl :Molar flow rate of Cl- (mol/s)

P:Pressure (kPa)

P0:Standard pressure (kPa)

Pconcentrate:Concentrate pressure (kPa)

Pfeed: Feed pressure (kPa)

Ppermeate: Permeate pressure (kPa)

Pfa:Feed pressure at actual conditions, kPa

Pfs:Peed pressure at standard conditions, kPa

Ppa:Permeate pressure at actual conditions, kPa

Pps:Permeate pressure at standard conditions, kPa

ΔPfba:Device pressure drop at actual conditions, kPa

ΔPfbs: Device pressure drop at standard conditions, kPa

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Qs: Permeate flow rate at standard conditions, m3/sec

Qa: Permeate flow rate at actual conditions, m3/sec

Qconcentrate : Volumetric flow rate of the concentrate stream (m3/s)

Qfeed: Volumetric flow rate of the feed stream (m3/s)

Qpermeate : Volumetric flow rate of the permeate stream (m3/s)

R:Universal gas constant

%SPa: Percentage salt passage at actual conditions

%SPs: Percentage salt passage normalized to standardconditions

TCFa: Temperature correction factor at actual conditions

TCFs: Temperature correction factor at standard conditions

T: Absolute temperature, K

T s : Standard feed water temperature (K)

T a Actual feed water temperature (K)

TDS: Total dissolved solids (mg NaCl/L)

vw: Molar volume of water (m3/mol)

Xw,feed :Mole fraction of water in the permeate stream (-)

Xw,permeate : Mole fraction of water in the feed stream (-)

Greek Letters

πfba: Feed–brine osmotic pressure at actual conditions, kPa

πfbs: Feed–brine osmotic pressure at standard conditions, kPa

πpa: Permeate osmotic pressure at actual conditions, kPa

πps: Permeate osmotic pressure at standard conditions, kPa

µw:Chemical potential of water (kJ/mol)

8. References:

[1].Al-Bastaki NM, A. A;” Predicting theperformance of RO membranes;” Desalination,132,181-187 (2000).

[2]. ASTM Standard ;”practice for standardizingreverse osmosis performance data, ASTM D4516,” (2006).

[3].Katchalsky A, Curran PF;Nonequilibriumthermodynamics in biophysics, Harvard UniversityPress, Cambridge.UK. (1965).

[4].Jonsson G, Boesen CE;”Water and solutetransport through cellulose acetate reverse osmosismembranes,”Desalination 17:145-165, (1975).

[5].Lewis GN;” The osmotic pressure ofconcentrated solutions, and the laws of perfectsolution,” J.Am.Chem.Soc. 30:668-683 (1908).

[6].M. Mulder , Basic Principles of MembraneTechnology, 2nd ed., pp. 280-307,Kluwer Acad.Publ., Dordrecht,Netherlands.(1996).

[7].Ari Seppälä;“Thermodynamic studies ofosmotic flows and their application to energyconversion systems,” Doctoral Dissertation,Helsinki University of Technology, (2007).

[8].Ari Seppälä, Lampinen MJ;” Thermodynamicoptimizing of pressure-retarded osmosis powergeneration systems,” Journal of Membrane Science161: 115-138 (1999)

[9].Kedem O, Katchalsky A;” Thermodynamicanalysis of the permeability of biologicalmembranes to non-electrolytes, Biochemical atBiophysical,” Acta 27:229-246 (1958).

[10].Weber WJ; Physicochemical processes forwater quality control., John Wiley and Sons, NewYork,US .(1972).

[11].WMD ;“plant Operational Reports” (2004-2005).

[12] .Zhao Y, T. J;"Assessment of ASTM D 4516for evaluation of reverse osmosis membraneperformance," Desalination, 180, 231-244 (2004) .

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International Journal of Engineering Research & Technology (IJERT)

Vol. 2 Issue 12, December - 2013

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ISSN: 2278-0181

www.ijert.orgIJERTV2IS121216