Emad Alhseinat and Roya Sheikholeslami

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1 An Application fo New Reliable Appoach to Pedict the Onset of Baite, Celestite and Gypsum Scaling duing Revese Osmosis Teatment fo Poduced Wate Emad Alhseinat and Roya Sheikholeslami Abstact Revese osmosis (RO) desalination is consideed a pomising solution fo Poduced Wate teatment and euse in the oil and gas industy. Howeve, scaling poblem is one of the main challenges that face Poduced Wate RO desalination. In this pape, the use of new completely theoetical appoach fo assessing fouling popensity along a full scale evese osmosis pocess is intoduced to pedict the onset of baite, celestite and gypsum scaling, as majo scaling salts facing the RO desalination of Poduced Wate. Thus, the scaling popensity of baite, celestite and gypsum have been assessed and discussed. Moeove, the effect of applied pessue, initial coss flow velocity, feed salinity and feed tempeatue on scaling popensity of baite, celestite and gypsum have been examined and discussed. This pocedue could be consideed as a vey impotant guideline, fo any attempt to use RO in Poduced Wate teatment. It could povide help on the specific design of RO pocess as well as in simulation of the opeating vaiables fo optimization of RO system. Index Tems Baite, celestite, poduced wate desalination, evese osmosis, scaling, scaling popensity. I. INTRODUCTION Poduced Wate is the wastewate poduced duing hydocabon (such as oil, gas and ta sand) pocessing. Desalination of Poduced Wate is attactive fo the oil and gas industy fo fou easons: Fistly, the substantial amount of globally Poduced Wate is estimated at aound 250 million baels/day compaed with aound 80 million baels/day of oil [1]. The amount of Poduced Wate makes it the lagest waste steam by volume associated with the oil and gas industy and makes the disposal of it a gave poblem and budens the opeations of oil and gas industy. Secondly, the high oveall cost of the Poduced Wate disposal as a esult of inceasing egulatoy constaints fo envionmental concens makes the economics unpalatable. Thidly, the fesh wate shotage especially in the aid egions like gulf egion, whee most oil and gas poduced counties ae located, makes the desalination of Poduced Wate beneficial, which could potentially help to educe the poblem of fesh wate scacity. Finally, the esults of ecent eseach studies by BP [2] show that injection of desalted wate to the oil well Manuscipt eceived Novembe 28, 2011; evised Decembe 8, This wok was suppoted in pat by the Univesity of Edinbugh. Emad Alhseinat is PhD student with the Institute of Mateials and Pocesses, School of Engineeing, The Univesity of Edinbugh, UK EH9 3JL ( e.alhseinat@ ed.ac.uk). Pofesso Roya Sheikholeslami is Chai of Chemical Pocess Engineeing at The Univesity of Edinbugh, UK EH9 3JL ( Roya.Sheikholeslami@ed.ac.uk). has a significant positive impact on the oil ecovey. Scale fomation seems to be a limiting facto in the desalination of Poduced Wate. Scaling limits the economic viability of Poduced Wate desalination by inceasing the enegy consumption and deceasing the efficiency of the pocess. Theefoe, scaling mitigation is cucial fo viably applying the desalination technology to the Poduced Wate. The fist step on scaling mitigation is the pediction of the scaling popensity fo specific feed wate within the RO unit. As detailed elsewhee [3] the cuent pactices fo assessing the scaling popensity ae based on empiical elationships that ae not suitable fo membane pocesses; the empiical indices used fo assessment the potential of pecipitation fouling which is commonly efeed to as scaling have been discussed in detail elsewhee []. In fact the complex chaacteistics of Poduced Wate exacebate the poblem of scaling because of the high pobability of scaling when salts such as CaCO3, CaSO, BaSO and SSO co-exist. Since, the mechanism of fouling is expected to be diffeent fo diffeent salts as it was demonstated [5] fo CaCO3 and CaSO. Sheikholeslami [6] and he eseach goup show that the co-existence of pecipitating salt affects the mechanism of fouling, the themodynamic, and kinetic behavio of each salt; hence, the single salt data is not applicable to the situation whee salts co-exist. Thus, the application of the cuent industial pactice fo assessing scaling popensity in desalination plant is limited and may only be able to cudely appoximate the scaling popensity fo simple feed wate at the entance to the RO desalination unit. Any effot fo scaling popensity pediction must conside the substantial change on system vaiables and paametes thoughout full scale RO desalination pocess and module. Indeed, the cuent pactices make thei pediction unealistic fo descibing full-scale RO pocess by assuming homogeneity along the membane channel, whee system vaiable and paametes change substantially along the long channel [7]. Theefoe, In this pape, the application of a newly developed eliable appoach [8] is intoduced fo assisting and pedicting the onset of scaling popensity duing full scale RO teatment of Poduced Wate. This new appoach is discussed in detail elsewhee [8] and is based on combining Sheikholeslami s Scaling Potential Index () [] with a simulation fo the behavio of salts concentation and pocess paametes within a long membane filtation channel. This eliable appoach has been used to study the effect of opeation vaiables, such as applied pessue and feed salinity, on onset of scaling popensity fo the majo scaling salts that could be faced duing Poduced Wate 5

2 desalination. The esults of this pape could be used as a guideline in the pocess design and opeation of a full scale RO fo Poduced Wate teatment. II. APPROACH DEVELOPMENT To accuately pedict the onset of scaling along a membane channel thee is a need to undestand and include the themodynamics of pecipitation, which signify the effect of tempeatue, pessue, co-pecipitation, high ionic stength and salinity on the solubility poduct. Then combining this solubility poduct with a simulation fo the behavio of ions concentation along the membane channel, which could be eached though using the pinciples of membane tanspot and mass consevation used by [8], [9] to pedict the vaiations of paametes in a long membane channel. Thus, using this fundamental pinciple; the effect of opeating paametes such as feed flow ate and the applied pessue on the onset of scaling popensity could be examined and veified. A. Themodynamic Analyses Baite, celestite and gypsum ae the common foms of baium sulphate, stontium sulphate and calcium sulphate that could be fomed in Poduced Wate desalination. Equations (1)-() epesent a geneal pecipitation eaction fo (MX) salt and the pecipitation eactions of baium sulphate, calcium sulphate and stontium sulphate, espectively. Scaling potential of any salt could be assessed using the Scaling Potential Index () as descibed in (5)-(8). was suggested and tested by Sheikholeslami [3],[]; in fact, Sheikholeslami with intoducing the has opened the way fo a completely theoetical assessment of the scaling popensity of any pecipitation salt, though a fundamental and unified appoach and by hypothesizing [10],[11],[12] the effect of poduct stuctue on Gibbs Fee Enegy of pecipitation eaction and hence the themodynamic solubility poduct () not being a constant and by incopoating the pinciples of themodynamics including Gibbs fee enegies as descibed below in (9)-(12) and the ion activities. M + X MX (s) (1) baite gypsum celestite Ba 2+ + SO BaSO (2) 2 Ca + + SO CaSO (3) S 2+ + SO SSO () IAP = log (5) 2 + = { Ca }.{ SO log baite 2+ = { Ca }.{ SO log gypsum { S }.{ SO = log celestiste } } 2 } (6) (7) (8) As seen in (5)-(8), calculation fo a tageted scaling salt equied the calculation of Ion Activity Poduct (IAP) as well as the themodynamic solubility constant () fo that salt as discussed befoe based on Sheikholeslami s appoach using Gibbs Fee Enegies of eaction as descibed below in (9)-(12); whee Δ G is Gibbs fee enegy of eaction, a is the activity, T is the absolute tempeatue (K) and R is the univesal gas constant. a e +. a M X Δ = Δ + G G RT ln (9) amx At equilibium, the Gibbs fee enegy of eaction ( Δ zeo. Thus, RT a. a G ) is M X ln = Δ G (10) a MX a By definition M a. a MX X = Fo pue MX pecipitation, the activity of MX ( a MX (11) ) is by convention unity and the standad Gibbs Fee Enegy of eaction ( Δ G ) is the diffeence between the total standad Gibbs fee enegy of fomation of poducts and the eactants [] and a function of pecipitate stuctue [10],[11],[12]. Δ G = ( Δ G ) ( Δ G ) (12) f poducts f eactants Fo accuate calculation of the along RO filtation channel, the effect of tempeatue and high ionic stength and salinity should be consideed and also combined with a simulation fo the change on concentations and opeational vaiables along the filtation channel. 1) Effect of tempeatue: Effect of tempeatue is incopoated as discussed elsewhee [] though the following equation fo RO pocesses. RT ln( ) = Δ G Δ H 298 T T T = Δ G (13) whee R is the gas constant ( kcal/mol.k), and T is the tempeatue (K); standad Gibbs fee enegies ae at 298K, (Δ H) is the enthalpy of eaction. 2) Effect of high ionic stengths and salinity: As the second autho oiginally intoduced fo RO desalination and discussed elsewhee [3],[],[13] Pitze model [1] was used to incopoate the high ionic stengths and salinity effects though the effects of ionic stength as well as the electostatic effects and shot and long ange ion inteaction foces. The IAP is calculated by using (1) and the activity coefficients (γ) ae calculated by using (15). IAP = a. a = γ [ M ] γ [ Z ] (1) M X M X γ = f (salinity and concentation of species) (15) 55

3 B. Modeling fo Vaiation of Concentation Along Membane Channel. The vaiation of ions concentations and the opeational vaiables in full scale RO is simulated by using finite diffeences fo solution of diffeential equations of tanspot and continuity along the module. The details of the mathematical model ae discussed elsewhee [8],[9]. In ode to simulate the effect of opeating paametes on onset of scaling popensity inside pactical RO systems; the effects of tempeatue, pessue, high ionic stength and salinity on wee examined. The paamete values used in these simulations wee eithe chosen fom the manufactues specifications o fom pactical opeating conditions as also used by othes [9], [7],[15]. This simulation is capable to pedict the effects of the vaiations of opeational paametes along membane channel on the onset of scaling tendency. Ou pocedue could be used as a vey impotant guideline, fo assessment of RO scaling in Poduced Wate teatment. It could guide on the specific design of RO pocess as well as in simulation of the opeating vaiables fo optimization of RO system. III. SIMULATIONS FOR THE ONSET OF SCALING Numeical simulations have been conducted to investigate the effect of opeating paametes on onset of scaling popensity pediction along a full-scale RO pocess. The confidence of the simulation has been discussed and shown elsewhee [8]. Unless othewise specified, Table 1 shows the values of the paametes that have been used in these simulations - these values wee also used by othes [9]. The simulation paametes can be adapted to specific opeating conditions and manufactue s design to suit any full scale RO pocess. TABLE I: PARAMETER VALUES FOR MODEL SIMULATIONS Length of RO system, L (m) 6 Channel height, H (m) 7 x 10 - Applied (pump) pessue, p 0 (Pa) x 10 6 Feed Salinity (mg/l) Coss flow velocity at entance (m/s) 0.1 Membane intinsic esistance, (Pa.s/m) 1.8 x Numbe of elements along RO system 00 Tempeatue, C 25 ph 7.1 Fiction coefficient due to spaces 5 Initial Ca 2+ concentation, ppm 1500 Initial SO 2-, ppm 2000 Initial Ba 2+, ppm 0.02 Initial S 2+, ppm 30 paticula simulation conditions the fo baite, celestite and gypsum wee negative fo the fist mete of the module and this means that onset of scale fomation will not be in the fist mete. Howeve, due to pemeation and concentation facto, scaling popensities of baite, gypsum, and celestite inceases aftewads and s becomes positive afte about 1.5m and inceases futhe along the channel. Fig.1. Onset of scaling popensity of Baite, Celestite and Gypsum along 6m membane channel. These obsevations demonstate that scaling is expected to take place afte 1.5m of the membane channel. B. Effect of Initial Coss Flow Velocity on Onset of Scaling Popensity fo Baite, Celestite and Gypsum. Fig. 2 shows the effect of vaious initial coss flow velocities on local values fo celestite along 6m RO pemeation channel. It is shown that coss flow velocity has vey impotant impact on localized scaling popensity. The same tends ae obseved fo the baite, celestite and gypsum scales, confiming that the lowest coss flow velocity esults in the highest scaling popensity. Thus, coss flow velocity should be optimized to educe scaling popensity fo all scaling salts. An optimum coss flow velocity can be obtained fom ou poposed appoach. As seen in Fig. 3, unde ou simulation conditions, opeating the RO at 0.2 m/s coss flow velocity will help to minimize baite, celestite and gypsum scaling. This obsevation could be used as a efeence to set the best possible opeational limit. A. Scaling Popensity of BaSO, SSO and CaSO along 6 m Membane Modules. The gives a consevative estimate fo assessing scaling potential; when is negative, cetainly no scale will fom; when zeo the system is at equilibium; when the is positive, the salt has a "potential" to fom scales []. Fig. 1 shows the onset of scaling popensity fo baite, celestite and gypsum in tems of. The figue shows that at these Fig.2. Effect of initial coss flow velocity on onset of scaling popensity fo Celestite. 56

4 Fig.3. Effect of initial coss flow velocity on onset of scaling popensity fo Gypsum, Baite and Celestite. C. Effect of Applied Pump Pessue on Onset of Scaling Popensity fo Baite, Celestite and Gypsum The applied pessue is one of the opeating vaiables that affect the onset of scaling popensity as shown fo gypsum in Fig. fo diffeent values of applied pessue. It is shown that inceasing applied pessue inceases the potential fo onset of scaling popensity. At the highest applied pessue, the membane is most pone to scale fomation unde othewise identical feed Poduced Wate conditions. Thus, theoetical assessment of the scaling potential enables one to opeate within the safe opeational limit. Fo example Fig. 5 shows opeating the RO at 5 ba will cetainly minimize the potential baite, celestite and gypsum scaling. D. Effect of Initial Feed Wate Salinity on Onset of Scaling Popensity fo Baite, Celestite and Gypsum. Salinity of feed wate to RO pocess vaies fom location to location and may also vay with time at a given location. Theefoe, Fig. 6 has been plotted to show the effect of salinity vaiation on scaling popensity of baite in tem of. It shows that at highe feed wate salinities, with othe paametes constant, the lowe would be onset of baite scaling popensity along the membane channel. Fig. 7 shows that at salinity of 0000 ppm no scales fom along the channel, since the cuves fo these thee salts (baite, celestite and gypsum) ae negative along the whole channel. The decease in the potential fo onset of scaling popensity along the pemeation channel, as the feed wate salinity inceases is due to the salinity effects on the pemeation diving foce as well on the ion activity. As the salinity inceases the net diving foce deceases and the pemeate flux will decease leading to decease in concentation facto and potential fo scaling. Anothe eason fo the decease in scaling popensity as a esult of an incease in feed salinity is the incease in solubility limit, due to the incease of non-scaling salt (NaCl) concentation [3],[], [11], [13]. It should be noted that in Fig. 6 and Fig. 7 the only vaiable between the cuves ae the concentation on non-scaling species; the concentation of scaling species and othe paametes ae the same in all the cuves on the figues. Fig.. Effect initial applied pump pessue on onset of scaling popensity fo Gypsum. Fig.6. Effect of feed salinity on onset of scaling popensity fo Baite. Fig. 5. Effect initial applied pump pessue on onset of scaling popensity fo Gypsum, Baite and Celestite. Fig.7. Effect of feed salinity on onset of scaling popensity fo Baite, Celestite and Gypsum. 57

5 E. Effect of Feed Wate Tempeatue on Onset of Scaling Popensity fo Baite, Celestite and Gypsum Figs show the scaling popensity () fo baite, celestite and gypsum along a 6m pemeation channel at diffeent feed wate tempeatues. The themodynamic solubility constant () was calculated by using (13) at the simulated tempeatues; the effect of tempeatue on density and viscosity was also taken into account. As seen in Fig. 8, inceasing tempeatue fom 15 o C to 70 o C appeas to educe the possibility of baite scale fomation. By opeating at 0 o C, baite scaling can be avoided. Fig. 9 shows that inceasing the tempeatue will incease the pobability of celestite pecipitation, as seen in Fig. 9 at 70 o C the cuve of celestite will be positive along the whole 6 m filtation channel. As seen in Fig. 10, inceasing tempeatue did not appeciably effect the scaling popensity of gypsum though gypsum is an invese solubility salt. The effect of tempeatue on membane popeties should be consideed and futhe attention should be given fo the effect of high tempeatues on the membane stuctue and pefomance as it could be one of the main dawbacks of opeating at high tempeatues. Fig. 10. Effect of feed tempeatue on onset of scaling popensity fo Gypsum. IV. CONCLUSION In this pape fundamental elationships wee used to eliably simulate and assess scaling popensity of baite, celestite and gypsum, along full-scale RO membane. This eliable simulation has been eached though incopoation of the localized paametes along the membane filtation channel in a fundamental and unified appoach fo assessing the scaling potential. This appoach allows studying the effect of hydodynamic and inteactive effects on scaling potential fom the fundamental pinciples. The effects of initial applied pessue, initial coss flow velocity, initial feed wate salinity and feed wate tempeatue on scaling popensity development of baite, celestite and gypsum wee investigated. The ability to simulate the opeational conditions and pefomance is impotant fo pocess design and fo opeating the RO unit within the most suitable opeational limit to avoid scaling. Fig. 8. Effect of feed tempeatue on onset of scaling popensity fo Baite. G IAP R T V. NOMENCLATURE standad Gibbs fee enegy ion activity poducts themodynamic solubility poduct the univesal gas constant scaling potential index tempeatue (K) Geek lettes α Δ Δ f G Φ Δ G Φ Δ H γ activity denote the diffeence between two values standad Gibbs fee enegy of fomation standad mola Gibbs fee enegy of eaction enthalpy of eaction activity coefficient Subscipts equilibium f fomation eaction Fig. 9. Effect of feed tempeatue on onset of scaling popensity fo Celestite. Supescipts 0 efeence value Φ efeence tempeatue value 58

6 REFERENCES [1] A. Fakhu'l-Razi, A. Pendashteh, L. C. Abdullah, D. R. A. Biak, S. S. Madaeni, and Z. Z. Abidin, "Review of technologies fo oil and gas poduced wate teatment," Jounal of Hazadous Mateials, vol. 170, pp , [2] P. L. McGuie, J. R. Chatham, F. K. Paskvan, D. M. Somme, and F. H. Caini, "Low Salinity Oil Recovey: An Exciting New EOR Oppotunity fo Alaska s Noth Slope," in SPE Westen Regional Meeting, [3] R. Sheikholeslami, "Assessment of the scaling potential fo spaingly soluble salts in RO and NF units," Desalination, vol. 167, pp , 200. [] R. Sheikholeslami, "Scaling potential index () fo CaCO3 based on Gibbs fee enegies," AIChE Jounal, vol. 51, pp , [5] Bansal.B, Mülle-Steinhagen.H, and D. Chen.X, "Effect of Suspended Paticles on Cystallization Fouling in Plate Heat Exchanges," TRANSACTIONS- AMERICAN SOCIETY OF MECHANICAL ENGINEERS JOURNAL OF HEAT TRANSFER, vol. 119, pp , [6] R. Sheikholeslami, Fouling in Membanes and Themal Units: A Unified Appoach- its Pinciples Assessment, Contol and Mitigation. L'Aquila, Italy: Balaban Desalination publications, pp , [7] K. L. Chen, L. Song, S. L. Ong, and W. J. Ng, "The development of membane fouling in full-scale RO pocesses," Jounal of Membane Science, vol. 232, pp , 200. [8] E. Alhseinat and R. Sheikholeslami, "A completely theoetical appoach fo assessing fouling popensity along full-scale RO pocess," Desalination. [9] L. Song, S. Hong, J. Hu, S. Ong, and W. Ng, "Simulations of Full-Scale Revese Osmosis Membane Pocess " Jounal of Envionmental Engineeing vol. 128, pp , Oct [10] R. Sheikholeslami, "Stategies and futue diections fo systematic chaacteization of feed wate and detemination of scaling limits and ates fo pocess assessment and optimization," pesented at the Intenational Desalination Association Wold Congess, Bahain, [11] R. Sheikholeslami, "Mixed salts--scaling limits and popensity," Desalination, vol. 15, pp , [12] R.Sheikholeslami, "Scaling of pocess equipment by saline steams - challenges ahead," Wate Science & Technology, vol. 9, pp , 200. [13] R. Sheikholeslami and H. W. K. Ong, "Kinetics and themodynamics of calcium cabonate and calcium sulfate at salinities up to 1.5 M," Desalination, vol. 157, pp , [1] R. T. Pabalan and K. S. Pitze, "Themodynamics of concentated electolyte mixtues and the pediction of mineal solubilities to high tempeatues fo mixtues in the system Na-K-Mg-Cl-SO-OH-H2O," Geochimica et Cosmochimica Acta, vol. 51, pp , [15] K. G. Tay and L. Song, "A moe effective method fo fouling chaacteization in a full-scale evese osmosis pocess," Desalination, vol. 177, pp ,