A fed-batch design approach of struvite system in ...

Chemical Engineering Science 61 (2006) 3951 – 3961 www.elsevier.com/locate/ces

A fed-batch design approach of struvite system in controlled supersaturation Md. Imtiaj Ali ∗,1 , Philip Andrew Schneider School of Engineering, James Cook University, Townsville, Queensland, QLD-4811, Australia Received 25 November 2004; received in revised form 23 August 2005; accepted 20 January 2006 Available online 20 March 2006

Abstract This paper focuses on struvite (MgNH4 PO4 · 6H2 O) crystallization in controlled supersaturation. Struvite can be used as a slow-release fertilizer. Crystallization experiments were conducted using supersaturated solutions. The secondary focus of this paper is the design of a struvite recovery system in fed-batch-controlled supersaturation mode. The design and commissioning of fed-batch struvite crystallization included the determination of operating supersaturation of struvite crystallization, suitable seed materials and the composition of feed solution. Determination of operating supersaturation of struvite crystallization was conducted by two steps including thermodynamic simulation using gPROMS2 (process simulation software) along with a set of batch experiments. Investigation of suitable seed materials was also conducted using set of batch experiments. Two types of seed materials including quartz sand and struvite seeds were used in the investigation of seed materials. Composition of feed solution included the investigation of struvite solution chemistry using PHREEQC3 thermodynamic modeling package. Based on the previously investigated design approach, struvite crystallization in fed-batch system was conducted using a 44-L of reactor with 15-L of initial reactant volume. 䉷 2006 Elsevier Ltd. All rights reserved. Keywords: Struvite; Control; Supersaturation; Reactor; Design; Thermodynamics

1. Introduction Magnesium ammonium phosphate hexa-hydrate (Mg NH4 PO4 · 6H2 O), commonly known as struvite, is a threat for nutrient-rich wastewater systems when the nutrient composition exceeds stable saturation limit (Muramatsu et al., 2000). Crystalline deposits of struvite characteristically form in the wastewater treatment infrastructure in high turbulence zone. The very sensitive zones of forming struvite are valves, bends in pipe, separating screens, pumps, etc. The mass of crystalline deposits formed can be extensive and can lead to operational failure by clogging water distribution pipes. Struvite satisfies

∗ Corresponding author. Tel.:+61 07 4781 43463; fax: +61 07 4775 1184.

E-mail address: [email protected] (Md.I. Ali). 1 After December 2006, corresponding email is [email protected]

(Dr. Philip A. Schneider), Tel.: +61 07 4781 5427. 2 gPROMS process simulation software, Process System Enterprise, London, UK. 3 US Geological Survey, Hydrologic Analysis Software Support Program, 437 National Centre, Reston VA 20192, E-mail: [email protected]. 0009-2509/$ - see front matter 䉷 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2006.01.028

a need of slow-release fertilizer and has many uses in horticulture such as in nurseries, golf course, etc. as boutique fertilizers (Nelson et al., 2000). The basic technique of growth type crystallizer was derived from mixed suspension mixed product removal (MSMPR). The key endeavor of this paper is to design a crystallization process capable of reducing spontaneous precipitation and allowing crystals to grow. Pilot scale-controlled crystallization, in the presence of suspended seeds, has been investigated in this paper. Thermodynamically, the metastable zone is defined as the critical zone of supersaturation of solution where crystallization is not governed by nucleation and thus avoids rapid and/or spontaneous precipitation. Hirasawa (1996) documented an experimental approach to determine metastable zone width for hydroxyapatite crystallization, relating to solution supersaturation, pH value, and minimum solubility limit of crystallization. Crystallization in the metastable zone is heterogeneous. In industrial crystallization, the metastable zone technique is widely practiced due to smoother continuous operation in controlled supersaturation (Thaller et al., 1981; McPherson, 1988; Srinivasakannan et al., 2002).

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2. Materials and methods This paper describes the fed-batch struvite crystallization in controlled supersaturation mode. Prior to the main experiment, preliminary experimental observations in batch scale along with thermodynamic simulation of struvite were required, which included the following investigations: • Determination of the metastable zone. • Determination of the suitable seed materials. • Determination of suitable composition of feed solutions.

Table 1 Thermodynamic equilibria of struvite solution chemistry (Childs, 1970; Smith and Martell, 1976; Martell and Smith, 1989; Morel and Hering, 1993) Thermodynamic equations

Value of K

{MgOH+ } ⇔ {Mg2+ } + {OH− } + {NH+ 4 } ⇔ {NH3 (aq)} + {H } 2− 3− + {HPO4 } ⇔ {H } + {PO4 } 2− + {H2 PO− 4 } ⇔ {H } + {HPO4 } {H3 PO4 } ⇔ {H+ } + {H2 PO− 4} 2+ } + {H PO− } {MgH2 PO+ } ⇔ {Mg 2 4 4 {MgHPO4 } ⇔ {Mg2+ } + {HPO2− 4 } 3− 2+ {MgPO− 4 } ⇔ {Mg } + {PO4 }

10−2.56 10−0.45 10−12.35 10−7.20 10−2.15 10−0.45 10−2.91 10−4.80

2.1. Determination of metastable zone The determination of the metastable zone includes the thermodynamic modeling along with a set of experimental observations. The determination of the metastable zone incorporates (i) determination of minimum struvite solubility limit (saturation) by thermodynamic modeling, (ii) determination of the minimum limit of spontaneous precipitation using a set of batch experiments.

pH Controller

pH probe

2.1.1. Determination of minimum struvite solubility limit The constituent chemicals of struvite are magnesium, ammonium and phosphate. Solution chemistry plays an important role in struvite formation. In supersaturated solution, struvite forms by chemical reaction of Mg2+ , NH4+ and PO3− 4 as demonstrated 3− Mg2+ + NH+ 4 + PO4 + 6H2 O ⇔ MgNH4 PO4 · 6H2 O.

Laser scattering device Sharp red line

Fig. 1. Schematic of experimental setup.

(1) The key parameters involved in struvite solution chemistry are solution supersaturation, pH, and concentration of reactants. Solution, consists of Mg2+ , NH4+ and PO3− 4 , remains in complex forms of Mg2+ , MgOH+ , MgH2 PO+ 4 , MgHPO4 , H3 PO4 , 2− 3− − H2 PO− , HPO , PO , MgPO , NH (Bouropoulos and Kout3 4 4 4 4 soukos, 2000). The basic thermodynamic relations of chemical complexes are listed in Table 1. The detailed description of struvite chemistry is previously published by Ali and Schneider (2005). Coding of thermodynamic equilibria in gPROMS determines the minimum pH limit of struvite saturation at the given range of concentrations of magnesium, ammonium and phosphate. The result of gPROMS thermodynamic simulation was then validated by the simple thermodynamic modeling using PHREEQC and the derived data from the Ohlinger (Ohlinger, 1999) solubility limit curve. 2.1.2. Determination of minimum limit of spontaneous precipitation The set of experiments were conducted using synthetic solution of 3, 4, 5 and 7 mM solution. Synthetic solution made up of de-ionized water was used in this experiment. Analytical grade MgCl2 and NH4 H2 PO4 was used to make up solution maintaining molar ratio of Mg : NH4 : PO4 as 1:1:1. The metastable zone experiment was conducted in batch-scale in

the absence of seed to identify the minimum pH of spontaneous precipitation at given concentrations. The schematic diagram of experimental setup is demonstrated in Fig. 1. Each experiment was conducted in a dark room using a laser light scattering device (laser pointer of helium neon laser light source) in which red color laser light passed through reactive solution of volume 1 l. The reactive solution, contained in 1.5 l clean glass beaker, was agitated using a rotational mechanical agitator of agitator blade diameter 2.5 in. A uniform speed of 35 rpm was employed throughout all experiments to provide sufficient mixing of solution. The initial pH of reactive solution (about 5.38) was slowly brought up to saturation level (investigated by thermodynamic modeling) using 0.5M of NaOH, and afterward 0.1M of NaOH was slowly added until the first appearance of crystal cloud in solution. In each experiment, a 30 min interval was allowed after each drop addition of NaOH to observe any alteration of reactive solution. By careful observation, the first appearance of crystal cloud was detected by laser light passing through the reactive solution. Scattering of laser light was applied parallel to the bottom and perpendicular to the surface of reactive solution container. To avoid any interference, laser light in solution must not touch the agitator blade. In the absence of any solid particles, undersaturated solution did not show any distinct redline of laser light, since

Md.I. Ali, P.A. Schneider / Chemical Engineering Science 61 (2006) 3951 – 3961

illumination of light depended on the reflection of light onto suspended particles present in solution. A distinct sharp red line in reactive supersaturated solution was formed due to the formation of spontaneous precipitation, which was visible from few meters distance in a dark room. This limit of pH is the apparent minimum pH for spontaneous precipitation. 2.2. Preparation of seed materials This paper incorporates two types of experiments, which include batch scale and fed-batch pilot scale experiments. Batch experiments (in the presence of seeds) were conducted to determine the suitable seed materials for struvite crystallization. Fed-batch scale crystallization was conducted in controlled supersaturation mode in the presence of previously investigated suitable seed materials. 2.2.1. Preparation of seed materials for batch experiments Preparation of seed (for batch experiment) was conducted in batch scale homogeneous struvite crystallization. In this circumstance, equimolar initial solution of 0.007 M of magnesium, ammonium and phosphate was used along with constant operating pH of 7.75. Generated struvite crystals were then used as seed to perform another experiment in heterogeneous crystallization mode (in the presence of seeds) until the desirable growth of struvite. This technique of seed preparation is called as serial seeding. Produced struvite crystal was then collected and sieved using 45–63
m of ASTM standard sieve. 2.2.2. Preparation of seed materials for fed-batch experiments Seed crystals for use in pilot scale experiments were prepared in fed-batch pilot scale crystallization using serial seeding technique (Section 2.2.1), as described in the previous paragraph. Size classified struvite seeds were prepared using 63–150
m of ASTM standard sieve.

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media and enhances crystal growth by layering of newly born crystallites onto the surface of the seeds. Sets of preliminary batch experiments were conducted using 3, 4 and 7mM concentration of solution with size classified (45–63
m) quartz sand and struvite seeds. Each experiment was conducted at previously investigated optimal supersaturation (close to saturation as previously investigated by thermodynamic simulation) with 12 h operational time. Sieving of seeds was conducted using ASTM standard sieve. The crystal size distribution (CSD) of seeds as well as growing struvite crystals was measured using Malvern particle-sizer. 2.4. Determination of feed composition To determine the concentration of different species of magnesium, ammonium and phosphate, concentration of requisite species along with the range of pH values were utilized as model input. All PHREEQC calculations were performed at 25 ◦ C temperature. Using the default database file, PHREEQC model calculated the concentrations of different complexes in3− cluding free Mg2+ , NH+ 4 and PO4 . To determine the saturation state of feed solution, concentration of the feed solution along with the range of pH value was utilized as model input. The default database of PHREEQC does not contain the equilibrium solid phase data of struvite; hence, it was necessary to code the equilibrium state of struvite together with the equilibrium constant. The thermodynamic data of struvite are available in the literature (Taylor et al., 1963; Ohlinger, 1999), and is shown in Eqs. (2)–(4). The detailed outcome of the PHREEQC thermodynamic modeling is illustrated in Section 3.2. It is worthwhile pointing out that the input data of heat transfer due to the reaction of struvite formation (H ) is used zero, since no such data are available in the available literature 3− MgNH4 PO4 · 6H2 O ⇔ 6H2 O + Mg2+ + NH+ 4 + PO4 ,

(2) 2.2.3. Sieving technique of crystals Wet sieving was employed to prepare size classified struvite seeds. Subsequent to the separation of crystals from crystal slurry, further separation of fines was employed using mild alkaline water (pH = 8.5) to avoid any dissolution of crystal (struvite) during sieving. Size classified crystal, obtained from wet sieving, was then air dried for 7-days to remove surface moisture. It is recommended to use the drying temperature of struvite in the range of 40–50 ◦ C. Experimental observations show that over drying (above 50 ◦ C) of struvite produces fragile crystals. Fragile seeds used in the fed-batch experiment may cause breakage of crystals due to recirculation of parent solution during crystallization. 2.3. Experiments on suitable seed materials As part of the design of the struvite reactor, it is necessary to determine suitable seed materials, which have positive influence on struvite growth. It is worthwhile pointing out that the presence of suitable seeds in reactive solution acts as diffusive

Log K = −13.27,

(3)

H = 0.

(4)

The saturation index (SI) identifies the saturation stage of solution, which is defined as SI = Log(IAP) − Log Ks ,

(5)

where Ks is the temperature corrected solubility constant. IAP the ion activity product of struvite. If SI > 0, supersaturation is indicated. When SI < 0, the solution is undersaturated with respect to the solid. When SI = 0, there is an apparent equilibrium with respect to the solid. 3. Results and discussion for design and commissioning of struvite reactor 3.1. Identification of metastable zone Thermodynamic equilibria of struvite chemistry were simulated using gPROMS and the simulated response was verified

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10.0

Labile supersaturation

9.5 9.0

Solution pH

8.5

Metastable Zone

8.0 7.5

Undersaturation

7.0 pH for minimum solubility by coded model

6.5

pH for minimum solubility using PHREEQC

6.0

pH for minimum solubility using Ohlinger's solubility curve

5.5

pH for rapid nucleation

5.0 0.001

0.002

0.003

0.004

0.005

0.006

0.007

Concentration of Magnesium, Ammonium and Phosphate (moles) Fig. 2. Identification of the metastable zone for struvite crystallization.

with thermodynamic modeling using the PHREEQC thermodynamics modeling package and the derived data of the Ohlinger (1999) solubility limit curve. Together with concentrations, the graphical presentation of experimental pH (Section 2.1.2) along with the simulated pH (Section 2.1.1) are presented in Fig. 2. The range of maximum and minimum pH limits (Fig. 2) is known as the operating pH range of struvite crystallization. Crystallization within that range is heterogeneous. Crystallization beyond this range is homogeneous, influenced by labile supersaturation. This operating zone of the crystallization is known as metastable zone. Crystallization, operating close to saturation, may intensify crystal growth by limiting nucleation. Crystal growth in this zone may be governed by surface diffusion (layering) of newly born crystallites onto seed particles.

3.2. Composition of feed solution Experimental control depends on correct stoichiometry of feed solution based on the fundamental reaction of struvite crystallization. The struvite reactor, operated at constant pH and constant reactant concentration was employed in this experimental control. Assume that the reactor is consisted of V l of reactive solution of x1 equimolar concentration of magnesium, ammonium and phosphate. The schematic diagram of the feed addition system is shown in Fig. 3. It is worthwhile pointing out that the feed solution must be split into two streams, since the presence of magnesium, ammonium and phosphate in the same stream may cause the formation of struvite crystal. Formation of struvite crystal in feed stream can adversely affect struvite process, leading to poor control. Fed-batch action of the process was maintained by the addition of two titrant streams, which comprised of x2 molar reactant concentration and x3 molar concentration of NaOH. Concentration of feed solution was maintained based on the stoichiometric formulation described in Eqs. (6) and (7)

Reactive Solution (Concentration = x molar) 1

Titrant- 2

Titrant-1

Fig. 3. Schematic diagram of feed addition.

Table 2 Possible composition of feed solution Feed Type

Titrant-1

pH

Titrant-2

pH

M1 M2 M3

MgCl2 + NaOH NH4 H2 PO4 + NaOH NH4 H2 PO4 + MgCl2

≈ 10–11 ≈ 9.0 ≈ 5.3

NH4 H2 PO4 MgCl2 NaOH

≈ 3.8 ≈ 7.0 ≈ 12.5

(Bouropoulos and Koutsoukos, 2000) x2 = Cx 1 + 2x1 ,

(6)

x3 = 2Cx 1 − 2x1 ,

(7)

where C is an arbitrary constant and 2x1 is considered for the dilution effect. Based on the feed addition rate in a trial fedbatch experiment, C = 10 was considered to maintain longer duration of experimental control. The combination of feed solution (titrant) has significant effect on robust and long-term experimental control of struvite crystallization. Possible compositions of feed solution using MgCl2 , NH4 H2 PO4 and NaOH are listed in Table 2. To evaluate the detailed solution chemistry of the various compositions (Table 2) of feed solution, thermodynamic modeling was conducted using PHREEQC thermodynamic modeling package. The model input was x2 = 0.06 M (MgCl2 and/or NH4 H2 PO4

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4MgCl2 + 8NaOH = OH− + H+ + H2 O + 8Cl− + Mg2+ + MgOH+ + 7Na+ + NaOH + O2 + Mg(OH)2 + MgO. (8) A thermodynamic modeling using PHREEQC shows that both Brucite [Mg(OH)2 ] and Periclase (MgO) remains in supersaturated state in solution at the specified solution concentration of 0.06 M of MgCl2 and 0.11 M of NaOH. It is worthwhile pointing out that the saturation indexes (SI) for Brucite are 23.09 and 37.02 at pH value of 4 and 11, respectively. Whereas, the saturation indexes (SI) for Periclase are 27.88 and 41.82 at pH 4 and 11, respectively, for the specified feed solution concentration described above. Moreover, magnesium may form some other complexes including MgOH+ and Mg(OH)2 (active), as computed by PHREEQC model (Eq. (8)). MgOH+ remains in dissolved form and not accountable for magnesium loss. Mg(OH)2 (active) is a solid phase, which remains in undersaturated form (SI = −40 at pH 4; SI = −22.50 at pH 14 at the specified feed concentration, described above). Therefore, Mg(OH)2 (active) also not accountable for magnesium loss in feed solution. Hence, the loss of reactive magnesium is actually caused by the solid formation (flock) of Brucite [Mg(OH)2 ] and Periclase (MgO). Thermodynamic prediction, using PHREEQC, demonstrates the trend of Mg2+ in titrant-1 when composition type M1 and M3 is maintained (Fig. 4). As described in Fig. 4, about 14% reduction (loss due to precipitation+reduction of concentration due to complex formation) of free magnesium occurs at pH 11, due to the formation of dissolved MgOH+ , as well as solid formation of Brucite [Mg(OH)2 ] and Periclase (MgO). A trial experiment showed that the precipitation of flock in the feed solution did not take part in feed addition since it had settled at the bottom in the static condition. At the make up pH of titrant-1 of feed type M1 , this precipitant caused loss of soluble magnesium in the form of Mg(OH)2 (and MgO) precipitate in the feed (less than 14%), leading to imbalance in reactant concentration in the system.

2+

% Free Magnesium ion (Mg )

100 80 Free Magnesium ion at pH value 5.3 is = 100% (M3: Titrant-1)

60 40

Free Magnesium ion at pH value 11 is = 86.06% (M : Titrant-1) 1

20 0 4

6

8

10

12

14

pH value

Fig. 4. Free Mg2+ concentration in feed-type M1 and M3 of titrant-1.

100 % Free Ammonium (NH4+ )

concentration), x3 =0.11 M (NaOH concentration), as described in Eqs. (6) and (7). It is worthwhile noting that the feed concentration of NaOH and MgCl2 and/or NH4 H2 PO4 was calculated considering the average reactive solution concentration of 0.006 M (x1 ) as employed in the fed-batch experiment (viz. Section 4). Titrant-2 of each feed type (Table 2) consists of single component, whereas titrant-1 of each feed type is multicomponents phase. Hence, there is high possibility of forming various complexes in titrant-1 of each feed type, leading to alteration of desired feed concentration. Therefore, the detailed solution chemistry of titrant-1 is described in the next paragraphs, since it is accountable for poorer experimental control. Eq. (8) shows the formation of the possible chemical components when MgCl2 and NaOH solution are mixed to make up titrant-1 (feed type 1 of composition types M1 and M3 ). Possible aqueous phases form in the feed solution are OH− , H+ , H2 O, Cl− , H2 , MgOH+ , Mg2+ , Na, NaOH and O2 , whereas the possible solid phases are Brucite [Mg(OH)2 ] and Periclase (MgO)

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Free Ammonium ion at pH value 5.4 is = 100% (M3: Titrant-1)

80 60

Free Ammonium ion at pH value 9.0 is = 72.91% (M2: Titrant-1)

40 20 0 4

6

8

10 pH value

12

14

Fig. 5. Free NH+ 4 and NH3 concentration in feed-type M2 and M3 of titrant-1.

Mixing of NH4 H2 PO4 and NaOH solution to make up titrant1 (feed type M2 ) led to imbalance control due to transformation + of NH+ 4 to volatile NH3 . The transformation of NH4 to NH3 caused significant loss of NH+ 4 in the form of NH3 (probably less than 27.09%) from the feed solution (viz. Fig. 5 at pH 9.0), leading to an alteration of the desired supersaturation in the reactive solution. Mixing of NH4 H2 PO4 and MgCl2 to make up titrant-1 of feed type M3 could easily form struvite in feed solution, since feed solution (titrant-1 of feed type M3 ) remained strongly concentrated by the feed magnesium, ammonium and phosphate. A trial experiment conducted with 0.006 M equi-molar concentration of reactive magnesium, ammonium and phosphate along with the equimolar feed concentration of 0.06 M of (Eqs. (6) and (7)) magnesium, ammonium and phosphate caused struvite precipitation in feed solution at pH value of 5.52. Thermodynamic modeling using PHREEQC indicated supersaturation of solution in this circumstance. Precipitation of struvite in feed solution caused the alteration of feed concentration very substantially leading to very weak experimental control. In this circumstance, reactive solution achieved undersaturation at the initial stage of experiment startup. Undersaturation of solution occurred due to the reduced feed concentration. Therefore, titrant-1 of feed type M3 caused significant impact on faulty

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Fig. 6. Scanning electron microscopic view of: (A) struvite seeds, (B) growing struvite.

Fig. 7. Scanning electron microscopic view of: (A) quartz sand seeds, (B) growing struvite with quartz sand.

experimental control leading to the change of supersaturated solution to the undersaturated. It is therefore imperative to use feed solutions according to M1 to minimize loss of reactants due to the formation of struvite and NH3 in feed solution. Continuous stirring of titrant 1 (feed type M1 ) set the feed solution in motion, thus avoiding the settlement of Mg(OH)2 precipitate. This well-mixed Mg(OH)2 in feed solution would then redissolve on entry to the reactor. The ionic strength of the reactive solution was altered maintaining 0.1 M of NaCl. Considering the dilution effect of the combined feed addition, 0.2 M of NaCl was required in titrant-2 of feed type M1 to maintain 0.1 M concentration of NaCl in the reactive solution. The addition of 0.2 M of NaCl reduced the pH value of titrant-2 (feed type M1 ) to 3.60. In this circumstance “acid–base neutralization” occurred in reactive solution when both titrants were added at the same flow rate to maintain constant stoichiometry. A trial experiment showed that acid–base neutralization caused excessive flow rate of feed solution, leading to the occurrence of extreme supersaturation. Adjustment of pH of titrant-2 (feed type M1 ) was thus required to minimize the acid–base neutralization effect. Experimental investigations showed that the pH difference between titrant-2 (type M1 ) and reactive solution 1.0–1.25 maintained reasonably consistent control of the process.

3.3. Determination of suitable seeds (batch scale experiment) Figs. 6 and 7 describe the scanning electron microscopic (SEM) view of struvite growth, using struvite seeds and quartz sand seeds, respectively. The SEM view in Fig. 6 demonstrates the similarity of typical orthorhombic shape of struvite seeds and growing struvite due to crystallization. Noticeable increase of size of struvite seeds was observed after crystallization (Fig. 6A and B). Quartz sand seeds did not take part in growth (Fig. 7); however, some struvite crystals formed due to the nucleation and subsequent growth of stable crystallites. The magnified view described in Fig. 8 shows the clear image of struvite growth in the presence of quartz sand seeds. Tiny dendrite-shaped struvite crystals form in the presence of quartz seeds and they remain as separate crystal instead of layering onto the surface of quartz sand. Particle size analysis using Malvern particle-sizer confirmed growth of struvite of about 14 and 4
m of growth for struvite seeds and quartz sand seeds, respectively (Figs. 9 and 10). The reason is probably the similarity of lattice structure between struvite seeds and newly born struvite crystallites enhances the diffusion integration process (Mullin, 1993). It is worthwhile pointing out that the crystallite is a domain of solid-state matter that has the same structure as single crystal. However, the

Md.I. Ali, P.A. Schneider / Chemical Engineering Science 61 (2006) 3951 – 3961

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likelihood of diffusion integration process for quartz seeds is less effective, leading to re-dissolving of unstable crystallites. It is worthwhile noting that the experiments relating to this section were conducted very close to saturation region, as investigated previously by thermodynamic modeling using PHREEQC. 4. Fed-batch-controlled crystallization Fed-batch struvite crystallization requires some pre-requisite observation for reasonably accurate control of supersaturation in fed-batch crystallization. 4.1. Summary of the design and commissioning Fig. 8. Magnified scanning electron microscopic view of struvite growth when quartz sand is used as seeds.

Based on the design and commissioning of fed-batchcontrolled struvite crystallization, following conclusions can

18 16 Struvite Seeds

14

Growing struvite (0.002M)

Volume %

12

Growing struvite (0.003M) Growing struvite (0.004M)

10

Growing struvite (0.005M)

8 6 4 2 0 0

50

100

150

200

Particle size in microns (1.2 to 600) Fig. 9. Crystal size distributions (CSD) of struvite seeds and growing struvite.

25 Quartz seeds

20

Growing struvite (0.002M)

Volume %

Growing struvite (0.003M)

15

Growing struvite (0.004M) Growing struvite (0.005M)

10

5

0 0

50

100

150

200

Particle size in microns (1.2 to 600) Fig. 10. Crystal size distributions (CSD) of quartz sand seeds and growing struvite with quartz sand.

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Data Logger

pH Controller Automatic temperature Control System Feed titrant-1

Feed titrant-2 Dosing Pump

Dosing Pump

Recirculation PVC pipe

Cooling Coil

Recirculation Pump Fig. 11. Schematic of controlled struvite crystallization.

be made:

4.2. Design of struvite reactor

• Fed-batch experiment is required to operate within previously investigated stable metastable zone to minimize spontaneous precipitation (Section 3.1). • The fed-batch system is required to maintain using two feed solutions. The first is a solution of NH4 H2 PO4 (the ammonia and phosphate source) and NaCl (to maintain ionic strength), pH adjusted to 6.0. The second is composed of NaOH (for pH maintenance) and MgCl2 (the magnesium source). Composition factors of the feed solutions are 12x1 (Mg2+ , NH+ 4 and PO3− 4 ) and 18x1 (NaOH), where x1 is the equimolar reactive solution concentration (Section 3.2). • Struvite crystal is required to use for struvite crystallization to enhance struvite growth rate (Section 3.3).

Unfortunately, no standard design approach exists for struvite crystallization. However, some common techniques such as suspension bed, seedings, feed addition are widely practiced (Bouropoulos and Koutsoukos, 2000; van der Houwen and Valsami-Jones, 2001; Kofina and Koutsoukos, 2003, 2005; Adnan et al., 2004). A schematic diagram of struvite fed-batch system is shown in Fig. 11. A struvite reactor of 44 L volume, made of clear Perspex, was used in this study (Fig. 12). The reactor was operated in suspended bed condition, allowing seeds and crystals to keep in uniform and full suspension. A pH controller (alpha 2000W) controlled the desired pH of solution and triggered feed addition when solution pH dropped below set point. Two dosing pumps (Grundfos DME-12) were operated for titrant (feed solution) additions based on two separate output signals sent via the pH controller. The pH controller was operated in pulse frequency control mode together with a proportional integral

Fed-batch experiment was conducted based on the previous investigation. The detailed design of fed-batch experiment is described in Section 4.1.

Md.I. Ali, P.A. Schneider / Chemical Engineering Science 61 (2006) 3951 – 3961

3959

Fig. 12. Photographic view of struvite reactor.

control strategy. Based on the pH sensor output, the dosing pump adjusted the flow rate of feed solution. The pH controller monitored and controlled the desired solution pH by two automatic relays (relay A and relay B) system, mounted in it. This system made the experimental control fully automatic. Therefore, the feed solution must be split into two parts as described in Section 3.2. It is worthwhile pointing out that the split of feed solution into three parts is not adaptable with a single pH controller due to the absence of third relay. Nonetheless, using more than one pH controller would be one way to maintain experimental control, which however could make the control system more complicated and more expensive (due to addition pH controller, dosing pump). The mixing of the reactive solution was carried out by a recirculation pump (Onga pump: model 413) together with solution recirculation loop, composed of PVC pipe of 1-in diameter. A variable transformer (variac) was used to maintain trouble free controlled flow rate of the recirculation pump. The top portion of PVC pipe was connected with an adjustable and flexible recirculation system (Fig. 12) to avoid short circuiting of recirculated streams. The reactor was seeded with 30 g of previously generated struvite crystal of size range 63–150
m. Samples of crystal suspension were collected through pump outlet and were filtered using 0.45
m filter paper. Continuous operation of the recirculation pump led to excessive temperature rise in the reactor. Significant increases of solution temperature caused offset (drift) of pH value. Therefore, it was necessary to maintain constant operating temper-

atures, enabling effective supersaturation control. A plasticcoated cooling coil (Figs. 11 and 12) assembled with control module was required to avoid corrosion of exposed copper and to maintain constant experimental temperature. The fed-batch experiment was conducted using the desired reactive concentration 0.0060 M along with the operating pH of 7.22. Previously generated fresh struvite seeds of mean diameter 127.77
m of weight 30 g were used in the fed-batch experiment. 5. Results and discussion from fed-batch experiment Followed by the experimental setup, as described in Figs. 11 and 12, the conducted experiment in controlled supersaturating showed the stability of control all through the experiment. During the fed-batch experiment seed crystals took part in growth. The experimental control of pH and reactant concentrations is shown in Section 5.1 and the growth of struvite crystals during crystallization is shown in Section 5.2. 5.1. Control of supersaturation in fed-batch experiment Fig. 13 illustrates the control profile of struvite system at different process conditions. Fig. 13(A) shows the profile of experimental control relating to the concentration of magnesium and phosphate as well as experimental pH during the experiment. Reasonably acceptable control of reactive solution composition and the operating pH of reactive solution were achieved in the fed-batch pilot scale experiment. Fig. 13(B) shows the

Md.I. Ali, P.A. Schneider / Chemical Engineering Science 61 (2006) 3951 – 3961 0.010 0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.000

8

6 5 4 Mg Conc (M) Average Controlled Mg (M) PO4 Conc (M) Average Controlled PO4 (M) pH value

3 2 1 0

0

2

(A)

4 6 Time in hours

8

10

30

8

6 20

5

15

4

10

Volume of reactive solution (L)

3

pH value

2

5

1

0

0 0

2

4

6

8

10

Time in hours

Fig. 13. (A) Process control of the system of expected reactive concentration 0.0045 M and pH 7.40. (B) Volume of reactive solution during the process control.

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Fig. 14. SEM view of produced struvite.

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increase of reactive volume due to the fed-batch action of experiment. After fed-batch experiment, generated struvite was separated from mother liquor using ASTM standard sieve of aperture 63
m. Collected struvite was air dried for seven days. Followed by air drying, dry sieving was implemented to separate fines of mean size smaller than 63
m. A recovery of 65–70% of the expected struvite mass was found in this experiment. For the perfect diffusion integration mechanism, it was expected that 100% of the generated struvite would be larger than 63
m (considering the lower size range of struvite seed). However, experimental observation showed that 30–35% of the generated struvite showed smaller size than was expected. Fractional generation of smaller struvite was due to the production of fines. Some possible reasons of fines generation may be due to the imperfect diffusion of newly born crystals, breaking of growing crystals/seeds due to the impeller action of recirculation system, etc. 5.2. Produced struvite crystal in fed-batch crystallization The controlled fed-batch experiment, described above, produced struvite crystal of orthorhombic shape (Fig. 14). CSD curve of seeds and growing struvite crystals, during the operation of pilot scale reactor operation, is shown in Fig. 15. The mean diameter of struvite seeds employed in the fedbatch experiment was 127.77
m and the mean diameter of

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Particle Size Range (microns) Fig. 15. CSD curve of struvite seeds and growing struvite.

harvested crystal was 161.63
m. Hence, the growth of crystal was 33.86
m. XRD analysis of the produced crystals indicates the crystals as struvite (Fig. 16). 6. Conclusions 1. The strategy described here has potential to cope with struvite crystallization at constant supersaturation, since pH and reactant concentration can be kept constant. 2. The fed-batch system must be maintained using two feed solutions. The first is a solution of NH4 H2 PO4 (the ammonia and phosphate source) and NaCl (to maintain ionic strength), pH adjusted to about 6.0. The second is composed of NaOH (for pH maintenance) and MgCl2 (the magnesium source). Composition factor of the feed solutions are 12x1 (Mg2+ , 3− NH+ 4 and PO4 ) and 18x1 (NaOH), where x1 is the equimolar reactive solution concentration. 3. It is possible to identify the metastable zone when the minimum struvite solubility limit (saturation limit) and the minimum limit of spontaneous precipitation are known. Laser light scattering into the reactor can detect the minimum limit of pH for spontaneous precipitation. Minimum struvite solubility limit is identified by thermodynamic simulation and

Md.I. Ali, P.A. Schneider / Chemical Engineering Science 61 (2006) 3951 – 3961

3961

900 800 struvite - File: 8057-01p general.raw Operations: X Offset -0.058 | Import

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Struvite, syn - NH4MgPO4·6H2O Dittmarite, syn - NH4MgPO4·H2O

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2-Theta - Scale Fig. 16. Analysis of struvite by XRD analysis.

was validated precisely by PHREEQC thermodynamic modeling package and the available solubility limit curve, proposed by Ohlinger (1999). 4. Uses of mother crystals (struvite as seeds) in struvite crystallization provide positive influence on struvite growth. Mother crystals provide support of efficient diffusion integration process, leading to faster growth of struvite during crystallization. 5. Produced struvite crystals show the typical orthorhombic shape with sharp edge. References Adnan, A., Dastur, M., Mavinic, S.D., Koch, F.A., 2004. Preliminary investigation into factors affecting controlled struvite crystallization at the bench scale. Journal of Environmental Engineering Science 3, 195–202. Ali, M.I., Schneider, P.A., 2005. Crystallization of struvite from metastable region with different types of seed crystal. Journal of Non-Equilibrium Thermodynamics 30 (2), 95–113. Bouropoulos, C.Ch., Koutsoukos, P.G., 2000. Spontaneous precipitation of struvite from aqueous solutions. Journal of Crystal Growth 213, 381–388. Childs, C.W., 1970. A potentiometric study of equilibria in aqueous divalent metal orthophosphate solutions. Journal of Inorganic Chemistry 9 (11), 2465–2469. Hirasawa, I., 1996. Study on the recovery of ions in wastewater by crystallization. Memoirs of the School of Science and Engineering 60, 97–119. Kofina, A.N., Koutsoukos, P.G., 2003. Nucleation and Crystal Growth of Struvite in Aqueous Media: New Prospective in Phosphorus Recovery. WASIC, Istanbul, Turkey.

Kofina, A.N., Koutsoukos, P.G., 2005. Spontaneous precipitation of struvite from synthetic wastewater solutions. Crystal Growth and Design 5 (2), 489–496. Martell, A.E., Smith, J.C., 1989. Critical Stability Constants. McPherson, A., 1988. The use of heterogeneous and epitaxial nucleants to promote the growth of protein crystals. Journal of Crystal Growth 90 (1–3), 47–50. Morel, F.M.M., Hering, J.G., 1993. Principles and Applications of Aquatic Chemistry. Wiley, New York. Mullin, J.W., 1993. Crystallization. third ed. Butterworth-Heinemann Publications, Ipswich, UK. Muramatsu, K., Yasui, A., Suzuki, T., Kiuchi, K., 2000. Factors involved in struvite formation by Natto bacilli. Biocontrol Science 51 (1), 57–60. Nelson, N.R., Mikkelson, R.L., Hesterberg, D.L., 2000. Struvite formation to remove phosphorus from anaerobic swine lagoon. In: Eighth International Symposium on Animal, Agriculture and Food Processing Wastes. Des Moines, USA, pp. 18–26. Ohlinger, K.N., 1999. Kinetics effects on preferential struvite accumulation in wastewater, Ph.D. Thesis, School of Science and Engineering, California State University. Smith, R.M., Martell, A.E., 1976. Critical Stability Constants. Plenum Publishers, New York. Srinivasakannan, C., Vasanthakumar, R., Iyappan, K., Rao, P.G., 2002. A study on crystallization of oxalic acid in batch cooling crystallizer. Chemical and Biochemical Engineering Quarterly 16 (3), 125–129. Taylor, A.W., Frazier, A.W., Gurney, E.L., 1963. Solubility product of magnesium ammonium. Transaction Faraday Society 59, 1580–1589. Thaller, C., Weaver, L., Eichele, G., Wilson, E., Karlsson, R., Jansonius, J., 1981. Repeated seeding technique for growing large single crystal structures of proteins. Journal of Molecular Biology 147, 465–469. van der Houwen, J.A.M., Valsami-Jones, E., 2001. The application of calcium phosphate precipitation chemistry to phosphorus recovery: the influence of organic ligands. Environmental Technology 22, 1325–1335.