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Ind. Eng. Chem. Res. 2004, 43, 1868-1874
Clean (“Green”) Ion-Exchange Technologies. 4. High-Ca-Selectivity Ion-Exchange Material for Self-Sustaining Decalcification of Mineralized Waters Process Dmitri Muraviev,*,† Ruslan Kh. Khamizov,‡ Nikolai A. Tikhonov,§ and Jaime Go´ mez Morales| Department of Chemistry, Autonomous University of Barcelona, E-08193 Bellaterra (Barcelona), Spain, Vernadsky Institute of Geological and Analytical Chemistry, Kosygin Str. 19, 117975 Moscow, Russia, Department of Mathematics, Physical Faculty, Lomonosov Moscow State University, 119899 Moscow, Russia, and Instituto de Ciencia de Materiales de Barcelona, Campus UAB, 08193 Bellaterra, Spain
This paper (the fourth in a series) reports the results of a theoretical and experimental study of the decalcification of seawater on different ion-exchange sorbents by simultaneous use of electroselectivity reversal and ion-exchange isothermal supersaturation (IXISS) effects. A detailed evaluation of the influence of the sorbent properties on the efficiency of the IXISS-based selfsustaining seawater decalcification process was carried out through a series of computer experiments using a mathematical model of the dynamics of ion exchange. It was found that the best sorbent to be used in the process is a modified A-type zeolite. The modification of the zeolite includes sequential treatment of the initial ion exchanger with dilute magnesiumcontaining solution (or seawater) and concentrated sodium salt solution. The first treatment was carried out at elevated temperature [15-20 °C higher than the temperature at which the modified zeolite (MZ) is expected to be used, Tex], and the second was performed at Tex. The complete regeneration of the MZ after the calcium sorption cycle was carried out with the calciumfree brine produced by the seawater desalination unit. The process is continuous and operates in the closed-cycle mode. Introduction Green chemistry is the use of chemistry for pollution prevention. More specifically, green chemistry is the design of chemical products and processes that are more environmentally benign. Green chemistry encompasses all aspects and types of chemical processes that reduce negative impacts to human health and the environment relative to the current state of the art. From this viewpoint, green ion-exchange technology (GIET) must be an ecologically clean and economically competitive alternative to existing large-scale industrial processes that do not satisfy modern ecological standards. The main drawback of the majority of known ion-exchange (IE) technologies is the large volume of aggressive liquid wastes (e.g., acidic and/or alkaline) that are produced during ion-exchanger regeneration. Hence, the enhancement of this particular stage must lead to a significant improvement of the overall process. A possible solution of this problem is the tailored application of the ionexchange isothermal supersaturation (IXISS) effect, discovered by Muraviev,1-4 for the conversion of the ion exchanger into the desired ionic form, i.e., for ionexchanger regeneration. The IXISS effect consists of the * To whom correspondence should be addressed. Tel.: 3493-5811836. Fax: 34-93-5812379. E-mail:
[email protected]. † Autonomous University of Barcelona. ‡ Vernadsky Institute of Geological and Analytical Chemistry. § Lomonosov Moscow State University. | Instituto de Ciencia de Materiales de Barcelona. Current address: Laboratorio de Estudios Cristalogra´ficos, IACT. CSIC-Universidad de Granada, Campus de Fuentenueva, s/n 18002 Granada, Spain.
formation of extremely stable supersaturated solutions (SSs) of either (1) the desired (product) or (2) the undesired (admixture) component in the IE column. These SSs demonstrate anomalous stability in being able to coexist with the granulated ion exchanger for a long period (from several hours to several days) without any change in concentration. After removal from the column, the SSs crystallize simultaneously. The use of the IXISS phenomenon to shift the equilibrium in IE systems in the desired direction has been shown to allow for the design of highly efficient and ecologically clean IE technologies, which can be referred to GIET-type processes. Three previous articles in this series were also focused on the development of GIET-type processes for (1) the ion-exchange synthesis of chlorine-free potassium fertilizers,9 (2) the recovery of high-purity magnesium compounds from seawater,10 and (3) the temperatureenhanced ion-exchange synthesis of potassium hydroxide.11 The present work reports the results on the further development of the GIET concept obtained by the development of an optimal ion-exchange material for the self-sustaining process of the decalcification of highly mineralized waters (seawater). A number of modern technologies require watertreatment processes that, in many instances, involve a calcium-removal stage.12-15 Sorption methods are widely applied for this purpose for low-mineralized surface waters. The problem of processing highly mineralized waters is much more complicated. An example is the preliminary treatment of seawater prior to its further desalination, where deep decalcification would solve the problems of gypsum core formation on distiller heater surfaces and membrane clogging in reverse-osmosis or
10.1021/ie030460b CCC: $27.50 © 2004 American Chemical Society Published on Web 03/20/2004
Ind. Eng. Chem. Res., Vol. 43, No. 8, 2004 1869 Table 1. Equilibrium Parameters of Separation of Ca2+ and Mg2+ Ions Determined in the Course of Zeolite Modification Cycles: Sorption from Seawater at 308 K, Desorption with 3 M NaCl at 298 K first cycle
a
second cycle
third cycle
parameter
sorption
desorption
sorption
desorption
sorption
desorption
C h Caa C h Mga RCa Mg
0.61 0.63 5.82
0.48 0.16 18.30
0.45 0.12 23.4
0.43 0.10 26.5
0.42 0.09 27.1
0.41 0.09 27.5
Concentration is given in milligram equivalents per cubic centimeter of sorbent bed.
electrodialysis devices. Modern seawater processing technologies, such as the ion-exchange recovery of magnesium,5,6,10 also require the preliminary removal of calcium. Decalcification of seawater can be successfully carried out by using a “self-sustaining” process16 based on the selective removal of calcium from seawater on an appropriate sorbent, followed by regeneration of the sorbent with Ca-free brine produced by a desalination (e.g., electrodyalisis) unit. The efficiency of the regeneration process is provided by the decrease of the sorbent selectivity toward Ca2+ compared to Na+ in a more concentrated solution with the same equivalent ratio of these ions because of the “electroselectivity reversal” effect.17,18 The simultaneous use of the IXISS and electroselectivity reversal effects within the regeneration stage can provide an additional shift of the IE equilibrium and, hence, increase the process efficiency. The specific requirements that sorption materials should have to create a competitive process of calcium removal from seawater are an extra-high selectivity toward Ca2+ over Mg2+ 5,6 and a low cost. The first requirement is dictated by the necessity of the efficient use of the sorbent capacity toward Ca2+ at a ∼5-fold excess of magnesium over calcium in the seawater under treatment. The second follows from the need to process 1000 m3 of seawater to produce 1 ton of magnesium.5,6,19 A theoretical analysis of the requirements for sorbents applicable in the decalcification of seawater was carried out by Barba et al.19 They showed that the efficiency of the process can substantially increase upon the use of sorbents with RCa Mg ) 20-25 and higher. Specially synthesized IE resins, namely, activite cationic resins, which are analogous to sulfonated poly(styrene-divinylbenzene) (PS-DVB) copolymers but contain more than one (1.5-2.0) sulfonic acid group per aromatic ring in their structure, were proposed for this purpose.19 These sorbents exhibit selectivity factor (RCa Mg) values from several tens to 100. However, these ion exchangers still remain quite costly and scarcely available. The other problems that limit the practical application of these materials are the difficulty of regenerating them with sodium salts because of the high selectivity for Ca2+ over Na+ and their low thermal, mechanical, and chemical stability. This paper reports the results obtained from the theoretical and experimental evaluation of the efficiencies of different IE materials when used in a selfsustaining process for the decalcification of seawater. It is shown that the best sorbent to be used in this process (in terms of both Ca versus Mg and Ca versus Na selectivities) is A-type zeolite modified with a dilute magnesium salt solution (or seawater). The combination of the electroselectivity reversal and IXISS effects acting in the same direction (in terms of the shift of IE equilibrium in the desired direction) allows for complete regeneration of the modified zeolite (after the calcium sorption cycle) with only Ca-free brine produced by the
seawater desalination unit. The process is shown to be continuous and to operate in the closed-cycle mode. Experimental Section Materials, Ion Exchangers, and Analytical Methods. Sodium, calcium, and magnesium salts of p.a. grade were used as received. A strong acid sulfonate cation-exchange resin KU-2 × 8 (the Russian analogue of Dowex-50 × 8 resin) with 8% cross-linking (DVB) and synthetic A-type zeolite (Nizhnii Novgorod, Russia) were commercial products. The total capacities of the ion exchangers were 4.5 (KU-2 × 8), and 4.0 (zeolite) mequiv/g. The concentrations of Na+, Ca2+, and Mg2+ were determined by the atomic absorption technique using a Saturn-5 (Russia) photometer. The relative uncertainty in the determination of the metal ions was no more than 2%. Zeolite Modification. The modification of the zeolite included sequential treatment of the initial ion exchanger with dilute magnesium-containing solution and concentrated sodium salt solution. The first treatment was carried out at elevated temperature [15-20 °C higher than the temperature at which the modified zeolite is expected to be used, Tex], and the second was performed at Tex. As seawater can be considered as a dilute solution of magnesium salt, the first treatment was also carried out using seawater. Zeolite modification with seawater was performed as follows: 35 dm3 of natural seawater (from Japan sea near Vladivostok city, east of Russia) preliminarily heated to 308 K was passed at a flow rate of ∼2 dm3/h (0.02 cm/s) through the thermostated column (L ) 33 cm, S ) 30.4 cm2, maintained at the same temperature) containing ω ) 1 dm3 of zeolite-A in the initial Na form. Then, the column was cooled to 298 K, and 5 dm3 of 3 M NaCl solution at the same temperature was passed through it at a flow rate of ∼1 dm3/h (0.01 cm/s). The zeolite capacities for Ca2+ and Mg2+ in the sorption and desorption stages were determined by measuring respective breakthrough curves of these components. The above sorption-desorption treatment stages were repeated two additional times. The modification of the zeolite was considered to be complete when the capacity values obtained in the sorption and desorption stages coincided with each other (see Table 1). Determination of IE Equilibrium Parameters. The equilibrium parameters of the ion exchanger were determined in the course of modification as follows: Sorption Stage. All seawater leaving the column during the sorption stage (35 dm3) was collected in a plastic tank along with the rinsing deionized water (5 dm3) passed through the column at the same temperature (308 K) right after achievement of ion-exchange equilibrium in the seawater treatment. The achievement of equilibrium was controlled by withdrawing a 10-cm3 portion every 30 min and analyzing the seawater macrocomponents (Na, Mg, and Ca) in this sample. IE
1870 Ind. Eng. Chem. Res., Vol. 43, No. 8, 2004
mCa ) CCa + CCaSO4 mMg ) CMg + CMgSO4 mSO4 ) CSO4 + CMgSO4 + CCaSO4 Ki,compl )
Results and Discussion Modeling and Computer Experiments. The detailed evaluation of the influence of the sorbent properties on the efficiency of the IXISS-based self-sustaining seawater decalcification process was carried out through a series of computer experiments using a mathematical model of the dynamics of ion exchange. The model describes a two-stage cyclic process shown schematically in Figure 1. In the first stage of each cycle, initial seawater (CΣ0 ) 0.43[Na+] + 0.12[Mg2+] + 0.02[Ca2+] ) 0.55 equiv/dm3) passes from the top to the bottom at a flow rate of ν through the sorbent bed of volume ω until the breakthrough of Ca2+. In the second stage, brine with the same ratio of seawater macrocomponents and a total concentration of CΣ0/(1 - φ) (where φ is the degree of recovery of freshwater from the seawater) passes in the opposite direction through the bed until complete (or partial) removal of Ca2+. The computer experiments were carried out until the stationary state was achieved, i.e., until the calcium sorption and desorption fronts coincided with each other. The model accounts for the mass-transfer and ion-exchange processes, formation of calcium and magnesium sulfates in the solution phase, and formation of supersaturated calcium sulfate solution during the regeneration stage as detailed below. (a) Equations Describing the Dynamics of the Process in the Column
∂mi ∂C hi ∂mi +ν + ) 0 i, j ) Na, Ca, Mg; 0 e x e l ∂t ∂x ∂t
∂mSO4 ∂t
+ν
∂mSO4 ∂x
mNa ) CNa
)0
CiSO4 CiCSO4
∂C hi h i) ) βi(µi - C ∂t
Figure 1. Schematic diagram of experimental pilot unit for selfsustaining seawater decalcination-desalination process (see text). 1, 1′, ion-exchange columns; 2, desalinator; 3, crystallizer.
equilibrium was considered to be achieved when the concentrations of the ions in the seawater entering and leaving the column were equal to each other. The IE equilibrium parameters (R and/or K ˜ , see below) for a given pair of ions were calculated from the results of the analysis of the initial seawater (C0,Na ) 0.43, C0,Mg ) 0.12, C0,Ca ) 0.02, C0,K ) 0.01 equiv/dm3) and that of the eluate passed through the column with zeolite under modification. The results of one series of these experiments are reported in Table 1. The calculation procedure is described in the Appendix. The decalcification process was tested on the setup shown schematically in Figure 1.
(1)
Ki
() Ci µi
1/zi
) Kj
() Cj µj
1/zj
ΣziC hi ) C hΣ Here, x is the coordinate along the column, l is the height of the sorbent bed, Ki,compl represents the association coefficients of the sulfates in the solution phase, is the porosity of the sorbent bed (fractional dead volume), Ki/Kj ) K ˜ ij represents the equilibrium coefficients in Nikolsky’s eq,20 zi represents the charge of ion i, βi represents the kinetic parameter of ion exchange for ion i, Ci(x,t) is the concentration of ionic component i in the solution phase, C h i(x,t) is the concentration of component i in the sorbent, µi(x,t) is the concentration of component i in the sorbent phase at equilibrium with Ci(x,t), and mi(x,t) is the total concentration of ions and complexes of component i in the solution. (b) Flow Rate Values
ν)
{
ν0
at Tn < t < Tn +
-ν0(1 - φ) at Tn +
T 2
T < t < T(n + 1) 2
(2)
Here, T is the duration of the whole cycle [the first half (T/2) is the time of the sorption stage, and the second half is the time of the regeneration stage], and n is the number of cycles (n ) 0, 1, 2, ...). (c) Boundary Conditions x)0 ) C0i Ci|Tn
